Add multidevs option to fsdev

multidevs specifies how to deal with multiple devices being shared with a 9p
export. `multidevs=remap` fixes the following warning:

```
9p: Multiple devices detected in same VirtFS export, which might lead to file
ID collisions and severe misbehaviours on guest!
You should either use a separate export for each device shared from host or
use virtfs option 'multidevs=remap'!
```

Signed-off-by: Julio Montes <julio.montes@intel.com>

qemu/qemu_test.go

@@ -149,6 +149,7 @@ func TestAppendDeviceFS(t *testing.T) {
 		SecurityModel: None,
 		DisableModern: true,
 		ROMFile:       "efi-virtio.rom",
+		Multidev:      Remap,
 	}
 
 	if fsdev.Transport.isVirtioCCW(nil) {