KVM: IOAPIC: only set remote_irr if interrupt was injected

There's a bug in the IOAPIC code for level-triggered interrupts. Its
relatively easy to trigger by sharing (virtio-blk + usbtablet was the
testcase, initially reported by Gerd von Egidy).

The "remote_irr" variable is used to indicate accepted but not yet acked
interrupts. Its cleared from the EOI handler.

Problem is that the EOI handler clears remote_irr unconditionally, even
if it reinjected another pending interrupt.

In that case, kvm_ioapic_set_irq() proceeds to ioapic_service() which
sets remote_irr even if it failed to inject (since the IRR was high due
to EOI reinjection).

Since the TMR bit has been cleared by the first EOI, the second one
fails to clear remote_irr.

End result is interrupt line dead.

Fix it by setting remote_irr only if a new pending interrupt has been
generated (and the TMR bit for vector in question set).

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>