KVM: PPC: Add KVM_CAP_NR_VCPUS and KVM_CAP_MAX_VCPUS

PPC KVM lacks these two capabilities, and as such a userland system must assume
a max of 4 VCPUs (following api.txt).  With these, a userland can determine
a more realistic limit.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>