vmgroups qemu command line is hardcoded

Looking at:

https://validation.linaro.org/scheduler/job/125333/log_file

Qemu was launched with:

qemu-system-arm -smp 2 -m 768 -cpu cortex-a15 -M vexpress-a15 -drive if=sd,cache=writeback,file=/lava-vm-images/vm1-job125332.1/kvm.img -nographic -enable-kvm -kernel /lava-vm-images/vm1-job125332.1/.zImage -append \\"root=/dev/mmcblk0p2 rw rootwait mem=768M console=ttyAMA0 38400n8\\" -dtb /lava-vm-images/vm1-job125332.1/vexpress-v2p-ca15-tc1.dtb'

This needs to be defined in job request instead of hardcoded in lava. The command line argumens to qemu change over time. For example in the current kvm job, we just moved from SD interface to virtio of rootfs:

https://git.linaro.org/qa/test-definitions.git/commitdiff/a24d57995fc0decb682e5548dbf3faca270987c3#patch2

In future we also want to test different solutions - for example nfsroot guests, virtio etc. Even more, we may want to launch guests using xen, libvirt or other tools.

- based on email discussion with me, milosz and Christopher.