pci-assign: limit # of msix vectors

KVM only supports MSIX table size up to 256 vectors,
but some assigned devices support more vectors,
at the moment attempts to assign them fail with EINVAL.

Tweak the MSIX capability exposed to guest to limit table size
to a supported value.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Gonglei <arei.gonglei@huawei.com>
Cc: qemu-stable@nongnu.org
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
