libxl: 9pfs has a QEMU backend

Add 9pfs to the kind of PV drivers that has a QEMU backend, specifically
to the macro QEMU_BACKEND.

This is needed otherwise upon domain destroy we get a timeout error:

libxl: error: libxl_device.c:1132:device_backend_callback: Domain 1:unable to remove device with path /local/domain/0/backend/9pfs/1/0
libxl: error: libxl_domain.c:1129:devices_destroy_cb: Domain 1:libxl__devices_destroy failed

This change should have been part of b53b4037ce "libxl/xl: add support
for Xen 9pfs".

Also add a comment in libxl_types_internal.idl to help remember changing
QEMU_BACKEND going forward.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>

tools/libxl/libxl_internal.h

@@ -636,6 +636,7 @@ typedef struct {
     (dev)->backend_kind == LIBXL__DEVICE_KIND_QDISK || \
     (dev)->backend_kind == LIBXL__DEVICE_KIND_VFB || \
     (dev)->backend_kind == LIBXL__DEVICE_KIND_QUSB || \
+    (dev)->backend_kind == LIBXL__DEVICE_KIND_9PFS || \
     (dev)->backend_kind == LIBXL__DEVICE_KIND_VKBD)
 
 #define XC_PCI_BDF             "0x%x, 0x%x, 0x%x, 0x%x"

tools/libxl/libxl_types_internal.idl

@@ -13,6 +13,7 @@ libxl__qmp_message_type = Enumeration("qmp_message_type", [
     (5, "invalid"),
     ])
 
+# Consider adding to QEMU_BACKEND in libxl_internal.h
 libxl__device_kind = Enumeration("device_kind", [
     (0, "NONE"),
     (1, "VIF"),