Now both classes (i8259, i8259-kvm) support this.  Move this upper to
the common class code.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20171210063819.14892-6-peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>