qemu-iotests: change _supported_proto to file for various tests

qemu-iotests: change _supported_proto to file for various tests

all these tests do anything of the following and thus fail with any
protocol other than file:
 - the tests use rm, cp or mv shell commands which only work on file
 - the tests use qcow2.py
 - the images construct new filenames (e.g. backing file names) and
   the logic is broken for anything else than file

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>