KVM: ioapic: merge ioapic_deliver into ioapic_service

Commonize the handling of masking, which was absent for kvm_ioapic_set_irq.
Setting remote_irr does not need a separate function either, and merging
the two functions avoids confusion.

Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>