core: Hot plug CPU - allow over-commit in QEMU limits

Allow to hot plug CPUs up to QEMU limits[1] even if they over-commit the
host resources.

[1] http://doc.opensuse.org/products/draft/SLES/SLES-kvm_sd_draft/cha.kvm.limits.html

Change-Id: Ic27aefa52543b201d018d07296de1e4983bfb863
Bug-Url: https://bugzilla.redhat.com/1097195
Signed-off-by: Roy Golan <rgolan@redhat.com>