virtio_pci: better macro exported in uapi

Macro VIRTIO_PCI_CONFIG assumes that userspace actually has a structure
with a field named msix_enabled.  Add VIRTIO_PCI_CONFIG_OFF that gets
the msix_enabled by value instead, to make it useful for userspace.  We
still keep VIRTIO_PCI_CONFIG around for now, in case some userspace uses
it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>