core: shceduling: destination host isn't guaranteed

when migrating VM to a specific host, or running once VM on
a specific host, and the specified host isn't suitable for running the VM,
another host could be selected. this patch fixes that, and
the operation would fail.

Change-Id: I6894c0d8d42bd57026ccc36b52d9d266add14384
Bug-Url: https://bugzilla.redhat.com/999630
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>