libvfio-user: update submodule to point to `spdk` branch

SPDK branch contains a fix for:
https://github.com/nutanix/libvfio-user/issues/790,
https://github.com/spdk/spdk/issues/3289


irqs_disable() out-of-boundary table access

Currently, it's not possible to use the main branch due to changes in
`libvfio-user` that make it incompatible with SPDK.

Change-Id: I63a881f9524a9c29669fccc083435977e0f20830
Signed-off-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/22604


Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: John Levon <levon@movementarian.org>
Reviewed-by: Jim Harris <jim.harris@samsung.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>