kvm: Use inhibit to prevent ballooning without synchronous mmu

Remove KVM specific tests in balloon_page(), instead marking
ballooning as inhibited without KVM_CAP_SYNC_MMU support.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>