KVM: VMX: Conditionally request interrupt window after injecting irq

If we're injecting an interrupt, and another one is pending, request
an interrupt window notification so we don't have excess latency on the
second interrupt.

This shouldn't happen in practice since an EOI will be issued, giving a second
chance to request an interrupt window, but...

Signed-off-by: Avi Kivity <avi@redhat.com>