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/provider/platform.go

@@ -32,7 +32,7 @@ func Get() (vmconfigs.VMProvider, error) {
 	logrus.Debugf("Using Podman machine with `%s` virtualization provider", resolvedVMType.String())
 	switch resolvedVMType {
 	case define.QemuVirt:
-		return new(qemu.QEMUStubber), nil
+		return qemu.NewStubber()
 	default:
 		return nil, fmt.Errorf("unsupported virtualization provider: `%s`", resolvedVMType.String())
 	}