core: live migrate - log error on failure

In case of shutting down the vm during snapshot
creation part of live migrate, an appropriate
failure AuditLog should be displayed
(in addition, the error should be logged).

Change-Id: Icae61b1c156e2bbb542a8790b310686028cc7a82
Bug-Url: https://bugzilla.redhat.com/957498
Signed-off-by: Daniel Erez <derez@redhat.com>