add the libvfio-user library as a submodule. build it as a meson
subproject.

libvfio-user is distributed with BSD 3-Clause license and
json-c with MIT (Expat) license

Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: c2adec87958b081d1dc8775d4aa05c897912f025.1655151679.git.jag.raman@oracle.com

[Changed submodule URL to QEMU's libvfio-user mirror on GitLab. The QEMU
project mirrors its dependencies so that it can provide full source code
even in the event that its dependencies become unavailable. Note that
the mirror repo is manually updated, so please contact me to make newer
libvfio-user commits available. If I become a bottleneck we can set up a
cronjob.

Updated scripts/meson-buildoptions.sh to match the meson_options.txt
change. Failure to do so can result in scripts/meson-buildoptions.sh
being modified by the build system later on and you end up with a dirty
working tree.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>