x86: re-inject emulated level pirqs in PV on HVM guests if still

x86: re-inject emulated level pirqs in PV on HVM guests if still
asserted

PV on HVM guests can loose level interrupts coming from emulated
devices if they have been remapped onto event channels.  The reason is
that we are missing the code to inject a pirq again in the guest when
the guest EOIs it, if it corresponds to an emulated level interrupt
and the interrupt is still asserted.

Fix this issue and also return error when the guest tries to get the
irq_status of a non-existing pirq.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>