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...