engine: Substract pendingVmSize from maxSchedulingMemory

maxSchedulingMemory should represent the amount of available
memory on host. If a migration is in progress we should
substract the memory that will be used by the VM in transition.

Change-Id: I87a0a77ae40049f18fb40d803ab58d5a7bda60f0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1083926
Signed-off-by: Martin Sivak <msivak@redhat.com>