fsdev: fix compilation with VIRTIO but not VIRTIO_9P

fsdev: fix compilation with VIRTIO but not VIRTIO_9P

hw/9pfs/Makefile.objs uses CONFIG_VIRTIO_9P to guard the definition for
FileOperations structs, while fsdev/Makefile.objs uses CONFIG_VIRTIO
to guard the use.  Mismatch causes linking to fail when CONFIG_VIRTIO
is set but CONFIG_VIRTIO_9P is not.

Fix it and use if/else to clarify that the two lines are for opposite
conditions.

Reported-by: Peter Maydell <[email protected]>
Fixes: b5dfdb082fc350f3e68dfa61dc988d97cad28cfe
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Acked-by: Greg Kurz <[email protected]>
Tested-by: Greg Kurz <[email protected]>
Signed-off-by: Greg Kurz <[email protected]>