kvm: move advertising of KVM_CAP_IRQFD to common code

POWER supports irqfds but forgot to advertise them.  Some userspace does
not check for the capability, but others check it---thus they work on
x86 and s390 but not POWER.

To avoid that other architectures in the future make the same mistake, let
common code handle KVM_CAP_IRQFD the same way as KVM_CAP_IRQFD_RESAMPLE.

Reported-and-tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Fixes: 297e2105
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>