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 <peter.maydell@linaro.org>
Fixes: b5dfdb082fc350f3e68dfa61dc988d97cad28cfe
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Greg Kurz <groug@kaod.org>