pci-assign: Enable MSIX on device to match guest

pci-assign: Enable MSIX on device to match guest

When a guest enables MSIX on a device we evaluate the MSIX vector
table, typically find no unmasked vectors and don't switch the device
to MSIX mode.  This generally works fine and the device will be
switched once the guest enables and therefore unmasks a vector.
Unfortunately some drivers enable MSIX, then use interfaces to send
commands between VF & PF or PF & firmware that act based on the host
state of the device.  These therefore may break when MSIX is managed
lazily.  This change re-enables the previous test used to enable MSIX
(see qemu-kvm a6b402c9), which basically guesses whether a vector
will be used based on the data field of the vector table.

Cc: qemu-stable@nongnu.org
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>