passthrough: Use physical INTX for virtual PCI functions

passthrough: Use physical INTX for virtual PCI functions

Allow virtual functions to use the legacy interrupt pin of the
corresponding physical device.  This is already half-implemented
by the code that uses pci_read_intx() in pass-through.c. This fills
in a missing piece.

An alternate solution is to reverse the previously applied pci_read_intx()
code and revert to always using INTA. However, that seems likely to lead to
excessive sharing of interrupt pins when multi-function pass-through is
used.

Tested-by: Tom Rotenberg <tom.rotenberg@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>