vfio/pci: use an invalid fd to enable MSI-X

vfio/pci: use an invalid fd to enable MSI-X

Guests typically enable MSI-X with all of the vectors masked in the MSI-X
vector table. To match the guest state of device, QEMU enables MSI-X by
enabling vector 0 with userspace triggering and immediately release.
However the release function actually does not release it due to already
using userspace mode.

It is no need to enable triggering on host and rely on the mask bit to
avoid spurious interrupts. Use an invalid fd (i.e. fd = -1) is enough
to get MSI-X enabled.

After dynamic MSI-X allocation is supported, the interrupt restoring
also need use such way to enable MSI-X, therefore, create a function
for that.

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>