core: UpdateVmCommand: Shared disk boot status

Check the boot status of a disk against all the VMs it's attached to,
not only the one its context is edited in.

Change-Id: I6a9d2e8800b9bf693fc55d8a627e0725277985ac
Bug-Url: https://bugzilla.redhat.com/854964
Signed-off-by: Allon Mureinik <amureini@redhat.com>