MC146818 RTC: add GPIO access to output IRQ

The MC146818 RTC device has output IRQ line. Currently the corresponding field
is only accessible through direct access. Such access violates Qemu model.

The patch makes the field accessible through GPIO. It also updates the setting
of the IRQ during initialization.

Signed-off-by: Efimov Vasily <real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>