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>