engine: Upper bound for VmPool.MaxAssignedVmsPerUser

Added an upper bound to the VmPool.MaxAssignedVmsPerUser property of
32,767 to prevent integer overflows and the possibility to get VmPool
in incosistent state by submitting exceedingly large value reulting in
negative value being set.

Change-Id: I1877312f61eae608103d596c90bffc6e6c3fa8d9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988056
Signed-off-by: Martin Betak <mbetak@redhat.com>