machine/linux: Support virtiofs mounts (retain 9p default)

I'm hitting a bug with 9p when trying to transfer large files.
In RHEL at least 9p isn't supported because it's known to have a
lot of design flaws; virtiofsd is the supported and recommended
way to share files between a host and guest.

Add a new hidden `PODMAN_MACHINE_VIRTFS` environment
variable  that can be set to `virtiofs` to switch to virtiofsd.

Signed-off-by: Colin Walters <walters@verbum.org>

contrib/cirrus/setup_environment.sh

@@ -410,7 +410,12 @@ case "$TEST_FLAVOR" in
         install_test_configs
         ;;
     machine-linux)
-        showrun dnf install -y podman-gvproxy*
+        showrun dnf install -y podman-gvproxy* virtiofsd
+        # Bootstrap this link if it isn't yet in the package; xref
+        # https://github.com/containers/podman/pull/22920
+        if ! test -L /usr/libexec/podman/virtiofsd; then
+            showrun ln -sfr /usr/libexec/virtiofsd /usr/libexec/podman/virtiofsd
+        fi
         remove_packaged_podman_files
         showrun make install PREFIX=/usr ETCDIR=/etc
         install_test_configs