msix: clear pending bit of an unused vector
PCI spec states:
if a masked vector has its Pending bit set, and the associated
underlying interrupt events are somehow satisfied (usually by software
though the exact manner is function-specific), the function must clear
the Pending bit, to avoid sending a spurious interrupt message later
when software unmasks the vector.
In our case this happens if vector becomes unused.
Clear pending bit in this case.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>