vfio: add device IO ops vector

vfio: add device IO ops vector

For vfio-user, device operations such as IRQ handling and region
read/writes are implemented in userspace over the control socket, not
ioctl() to the vfio kernel driver; add an ops vector to generalize this,
and implement vfio_device_io_ops_ioctl for interacting with the kernel
vfio driver.

Originally-by: John Johnson <john.g.johnson@oracle.com>
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250507152020.1254632-11-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>