xen: test for driver=tap2 sub-type in xen-xm

tap2 only handles 'aio', but not 'raw', which must be explicitly given:

XML:raw needs to be translated to XM:aio for 'tap' and 'tap2' Xen
drivers.

tests/xmconfigdata/test-disk-drv-blktap-raw.cfg

@@ -0,0 +1,12 @@
+name = "XenGuest1"
+uuid = "c7a5fdb0-cdaf-9455-926a-d65c16db1809"
+maxmem = 579
+memory = 394
+vcpus = 2
+localtime = 0
+on_poweroff = "destroy"
+on_reboot = "restart"
+on_crash = "restart"
+vif = [ "mac=00:16:3e:66:94:9c,bridge=br0,script=vif-bridge" ]
+bootloader = "/usr/bin/pygrub"
+disk = [ "tap:aio:/root/some.img,xvda,w" ]