blktapctrl: Select backend by prefix

This patch adds support for specifying the backend (tapdisk or ioemu)
to blktapctrl. Images can be specified e.g. as tap:tapdisk:aio,
tap:ioemu:qcow2 or tap:vmdk. When omitting the backend, a default is
chosen based on the image type (currently always tapdisk because ioemu
as a backend is broken until a follow-up patch series against qemu-xen
is applied)

Signed-off-by: Kevin Wolf <kwolf@suse.de>

tools/python/xen/xend/server/BlktapController.py

@@ -15,7 +15,8 @@ blktap_disk_types = [
     'qcow',
     'qcow2',
 
-    'ioemu'
+    'ioemu',
+    'tapdisk',
     ]
 
 class BlktapController(BlkifController):