backend: remove vm check from LiveMigrateDiskCommand

This patch shifts the VM status check from the LiveMigrateDiskCommand
to VmReplicateDiskStartTaskHandler.

This will:
* prevent CreateImagePlaceholderTaskHandler from failing if the VM
  crashes during its execution
* shift the failure to the appropriate handler (VmReplicateDiskStart)
* allow the CreateImagePlaceholderTaskHandler compensation to take
  place and remove the destination image when necessary

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=957703
Change-Id: Iac42a5d10373a0c1fcf9a224fb902f0ce481d3b7
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>