virtio-pci: guest notifier mask without non-irqfd

non-irqfd setups are currently broken with vhost:
we start up masked and nothing unmasks the interrupts.
Fix by using mask notifiers, same as the irqfd path.

Sharing irqchip/non irqchip code is always a good thing,
in this case it will help non irqchip benefit
from backend masking optimization.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>