KVM: Bump maximum vcpu count to 64

With slots_lock converted to rcu, the entire kvm hotpath on modern processors
(with npt or ept) now scales beautifully.  Increase the maximum vcpu count to
64 to reflect this.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>