If your disk XML contains <driver name='qemu'/>, which your earlier example did, then the conversion is correct since it maps to backendtype=qdisk. If however your XML contains <driver name='phy'/> and it maps to backendtype=qdisk, then we have a bug.
my native configuration has: "phy:/dev/mapper/keys,xvdz,r" and virsh domxml-from-native ... has converted ist to <driver name='qemu'/> so I think this is a bug...
