It was only for userspace i8259.  Move it to general code so that
kvm-i8259 can also use it in the future.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20171210063819.14892-4-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>