core: remove unused VdcBllMessage

Following introduced infrastructure of VM devices in
'next run' snapshot, removed now unused VdcBllMessage:
VM_CANNOT_UPDATE_DEVICE_VM_NOT_DOWN

Change-Id: I797de9140aabe5afe19c9655311b725c0b589ad5
Bug-Url: https://bugzilla.redhat.com/1035289
Signed-off-by: Daniel Erez <derez@redhat.com>