PowerMax driver may deadlock moving volumes between SGs

There's a potential deadlock scenario in PowerMax's masking.py move_volume_between_storage_groups" method.

The method uses 2 locks, one for the source Storage Group and another for the destination Storage Group, and it could happen that if 2 requests going in opposite directions are received simultaneously their first lock acquisition interleaves resulting in a deadlock situation.

    @coordination.synchronized(
        "emc-sg-{source_storagegroup_name}-{serial_number}")
    @coordination.synchronized(
        "emc-sg-{target_storagegroup_name}-{serial_number}")
    def move_volume_between_storage_groups(
            self, serial_number, device_id, source_storagegroup_name,
            target_storagegroup_name, extra_specs, force=False,
            parent_sg=None):

The scenario would be like this:

- User requests an instance migration from A to B
- User requests an instance migration from B to A
- Driver acquires the first lock for A-to-B which is something like cinder-emc-sg-SGA-###
- Driver acquires the first lock for B-to-A which is something like cinder-emc-sgSGB-###

The deadlock happens because A-to-B waits forever for the lock held by the B-to-A operation, which in turn cannot proceed because it’s waiting for lock help by A-to-B.