iommu: Fix hvm_dpci_eoi for smp guest in ioapic interrupt mode

iommu: Fix hvm_dpci_eoi for smp guest in ioapic interrupt mode

My testing shows, for some smp guests, ent->fields.mask will not be
unmasked when hvm_dpci_eoi() is invoked at the first time. In that
case, we should rely on timeout call back to do pirq_guest_eoi() and
should not stop the per-irq timer too early.

From: Wei Wang <wei.wang2@amd.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>