9p: simplify source file selection

Express the complex conditions in Kconfig rather than Makefiles, since Kconfig
is better suited at expressing dependencies and detecting contradictions.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Kconfig.host

@@ -28,6 +28,7 @@ config VHOST_USER
 
 config XEN
     bool
+    select FSDEV_9P if VIRTFS
 
 config VIRTFS
     bool

fsdev/Makefile.objs

@@ -1,6 +1,6 @@
 # Lots of the fsdev/9pcode is pulled in by vl.c via qemu_fsdev_add.
 # only pull in the actual 9p backend if we also enabled virtio or xen.
-ifeq ($(call land,$(CONFIG_VIRTFS),$(call lor,$(CONFIG_VIRTIO_9P),$(CONFIG_XEN))),y)
+ifeq ($(CONFIG_FSDEV_9P),y)
 common-obj-y = qemu-fsdev.o 9p-marshal.o 9p-iov-marshal.o
 else
 common-obj-y = qemu-fsdev-dummy.o

hw/9pfs/Kconfig

@@ -1,4 +1,9 @@
+config FSDEV_9P
+    bool
+    depends on VIRTFS
+
 config VIRTIO_9P
     bool
     default y
     depends on VIRTFS && VIRTIO
+    select FSDEV_9P