virttest.storage: Pylint/Eclipse style fixes

No actual changes, just cleanups.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.storage: Change default location

Use root_dir/$backup_dir in case $backup_dir is not absolute path
and change the default backup location to $autotest_data/images/.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.storage: Don't overwrite backup image during preprocess

Allow backup_image to skip_existing files and use this during
preprocess_image. This could save a lot of time with intrusive tests.

The idea is to create backup image only once and in case of
intrusive tests use restore_image_after_testing_on_error.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>