tap2 only handles 'aio', but not 'raw', which must be explicitly given:
| $ virsh domxml-to-native yyy.xml > yyy.xm
| $ xm new yyy.xm
| Error: tap:/srv/xen/xxx.img not a valid disk type
| $ sed -i -e 's/tap2:/&aio:/' yyy.xm
| $ xm new yyy.xm
Fix reading and writing "xen-xm" format for "tap2" by handling it the
same as "tap".