core: calculating the maxVcpus in the limits of qemu

calulate the right maxVCpus we send to VDSM to be in the limits of qemu
max number of sockets.

this eliminates the need to send an extra argument to VDSM
maxNumberOfSockets and eliminates an API change.

Change-Id: I452d0614406ed5e9e20ac3212a5d07e7a4e4b269
Bug-Url: https://bugzilla.redhat.com/1070890
Signed-off-by: Roy Golan <rgolan@redhat.com>