ceph-relation-changed hook fails due to hypervisor connection failure

## Issue
libvirt fails to start in Precise-Icehouse deployments (stable charm set). Restarting libvirt AND nova-compute was necessary for me, as nova-compute had also died. http://paste.ubuntu.com/10768920/

## nova-compute juju unit log
2015-04-07 21:29:17 INFO unit.nova-compute/0.ceph-relation-changed logger.go:40 subprocess.CalledProcessError: Command '['virsh', '-c', 'qemu:///system', 'secret-list']' returned non-zero exit status 1

## libvirtd.log from nova compute node
2015-04-07 21:26:22.456+0000: 7508: info : libvirt version: 1.2.2
2015-04-07 21:26:22.456+0000: 7508: error : virDriverLoadModule:79 : failed to load module /usr/lib/libvirt/connection-driver/libvirt_driver_storage.so /usr/lib/libvirt/connection-driver/libvirt_driver_storage.so: undefined symbol: rbd_create3

## Original description
When running mojo spec specs/full_stack/next_deploy/icehouse on precise the ceph-relation-changed hook fails with:

error: failed to connect to the hypervisor
error: no connection driver available for qemu:///system
Traceback (most recent call last):
  File "./hooks/ceph-relation-changed", line 335, in <module>
    main()
  File "./hooks/ceph-relation-changed", line 329, in main
    hooks.execute(sys.argv)
  File "/var/lib/juju/agents/unit-nova-compute-1/charm/hooks/charmhelpers/core/hookenv.py", line 544, in execute
    self._hooks[hook_name]()
  File "/var/lib/juju/agents/unit-nova-compute-1/charm/hooks/charmhelpers/core/host.py", line 312, in wrapped_f
    f(*args, **kwargs)
  File "./hooks/ceph-relation-changed", line 261, in ceph_changed
    key=relation_get('key'))
  File "/var/lib/juju/agents/unit-nova-compute-1/charm/hooks/nova_compute_utils.py", line 451, in create_libvirt_secret
    if secret_uuid in check_output(['virsh', '-c', uri, 'secret-list']):
  File "/usr/lib/python2.7/subprocess.py", line 544, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['virsh', '-c', 'qemu:///system', 'secret-list']' returned non-zero exit status 1

Restarting libvirt-bin solves the problem.

Additional info:

root@juju-lytrusty-machine-14:~# cat /var/log/libvirt/libvirtd.log
2015-03-03 11:20:20.609+0000: 7494: info : libvirt version: 1.2.2
2015-03-03 11:20:20.609+0000: 7494: error : virDriverLoadModule:79 : failed to load module /usr/lib/libvirt/connection-driver/libvirt_driver_storage.so /usr/lib/libvirt/connection-driver/libvirt_driver_storage.so: undefined symbol: rbd_create3
2015-03-03 11:20:20.617+0000: 7494: error : virDriverLoadModule:79 : failed to load module /usr/lib/libvirt/connection-driver/libvirt_driver_qemu.so /usr/lib/libvirt/connection-driver/libvirt_driver_qemu.so: undefined symbol: virStorageFileStat
2015-03-03 11:20:41.866+0000: 8506: info : libvirt version: 1.2.2
2015-03-03 11:20:41.866+0000: 8506: error : virDriverLoadModule:79 : failed to load module /usr/lib/libvirt/connection-driver/libvirt_driver_storage.so /usr/lib/libvirt/connection-driver/libvirt_driver_storage.so: undefined symbol: rbd_create3
2015-03-03 11:20:41.872+0000: 8506: error : virDriverLoadModule:79 : failed to load module /usr/lib/libvirt/connection-driver/libvirt_driver_qemu.so /usr/lib/libvirt/connection-driver/libvirt_driver_qemu.so: undefined symbol: virStorageFileStat
2015-03-03 11:22:51.725+0000: 8509: error : do_open:1170 : no connection driver available for qemu:///system
2015-03-03 11:22:51.726+0000: 8506: error : virNetSocketReadWire:1454 : End of file while reading data: Input/output error
2015-03-03 11:23:17.691+0000: 8516: error : do_open:1170 : no connection driver available for qemu:///system
2015-03-03 11:23:17.792+0000: 8506: error : virNetSocketReadWire:1454 : End of file while reading data: Input/output error
2015-03-03 12:18:19.220+0000: 8511: error : do_open:1170 : no connection driver available for qemu:///system
2015-03-03 12:18:19.221+0000: 8506: error : virNetSocketReadWire:1454 : End of file while reading data: Input/output error