qemu.tests.cfg: add remove_image_stg for usb_storage

need to remove the origin image and create a new image for format_disk
when running every usb_storage subcase.

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #1263 from spcui/usb_storage_fix

qemu.tests.cfg: add remove_image_stg for usb_storage