iotests: Allow check -o data_file

iotests: Allow check -o data_file

The problem with allowing the data_file option is that you want to use a
different data file per image used in the test.  Therefore, we need to
allow patterns like -o data_file='$TEST_IMG.data_file'.

Then, we need to filter it out from qemu-img map, qemu-img create, and
remove the data file in _rm_test_img.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Message-id: 20191107163708[email protected]
Signed-off-by: Max Reitz <[email protected]>