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>

pkg/machine/qemu/command/qemu_command_test.go

@@ -53,6 +53,8 @@ func TestQemuCmd(t *testing.T) {
 
 	expected := []string{
 		"/usr/bin/qemu-system-x86_64",
+		"-object",
+		"memory-backend-memfd,id=mem,size=2048M,share=on",
 		"-m", "2048",
 		"-smp", "4",
 		"-fw_cfg", fmt.Sprintf("name=opt/com.coreos/config,file=%s", ignPath),