Fix mixed pvm/hvm clusters and instance listing

The current implementation of the combining of the instance lists will
only do this for instances whose all four-fields match in both
hypervisors; however, this is broken for the dynamic fields (state,
times) which can change between the invocations of the two different
hypervisors if the instance is busy.

The patch checks only the memory and VCPUs, and makes mixed clusters
work even with 100% CPU instances.

Reviewed-by: imsnah