core: Move lock RunVmCommand -> RunVmCommandBase

Move RunVmCommand's exclusive lock up to its ancestor, RunVmCommandBase.

RunVmCommand and all it's extending classes' locking will remain
unchanged (as they inherit the lock from RunVmCommandBase), and the
migration flows will also benefit from the lock.
This is done in order to solve a race in virt/storage migration flows,
like the bug mentioned below, and makes sense regardless - you can't
start running a VM, e.g., if it's in the middle of migrating to another
host.

Change-Id: I8cbea4ae3a8bc47b4bbc451d8ecd3080287a3df0
Bug-Url: https://bugzilla.redhat.com/878131
Signed-off-by: Allon Mureinik <amureini@redhat.com>