docs: add vfio-user documentation

Add some basic documentation on vfio-user usage.

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-19-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
2 files changed