libxl: 9pfs has a QEMU backend

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 b53b4037cef6 "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>