The physical function defaults to operating in "PXE mode" after a
power-on reset.  In this mode, receive descriptors are fetched and
written back as single descriptors.  In normal (non-PXE mode)
operation, receive descriptors are fetched and written back only as
complete cachelines unless an interrupt is raised.

There is no way to return to PXE mode from non-PXE mode, and there is
no way for the virtual function driver to operate in PXE mode.

Choose to operate in non-PXE mode.  This requires us to trick the
hardware into believing that it is raising an interrupt, so that it
will not defer writing back receive descriptors until a complete
cacheline (i.e. four packets) have been consumed.  We do so by
configuring the hardware to use MSI-X with a dummy target location in
place of the usual APIC register.

Signed-off-by: Michael Brown <mcb30@ipxe.org>