core: Decrement pending memory/cpus after finished migration

The engine leaved pending memory in DB after successfull and
some unsuccessfull migrations and this caused hosts to reject
more VMs after some time.

Change-Id: Ic5ce2756d17a7bc3b0a5be56a44936a70aa227c5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1039900
Signed-off-by: Martin Sivak <msivak@redhat.com>