qemu-iotests/059: Fix leaked image files

qemu-iotests 059 left a whole lot of image files behind in the scratch
directory because VMDK creates additional files for extents and cleaning
them up requires the original image intact (it parses qemu-img info
output to find all extent files), but the image overwrote it many times
like it works for all other image formats.

In addition, _use_sample_img overwrites the TEST_IMG variable, causing
new images created afterwards to reuse the name of the sample file
rather than the usual t.IMGFMT.

This patch adds an intermediate _cleanup_test_img after each subtest
that created an image file with additional extent files, and also after
each use of a sample image. _cleanup_test_img is also changed so that it
resets TEST_IMG after a sample image is cleaned up.

Note that this test was failing before this commit and continues to do
so after it. This failure was introduced in commit 9877860 ('block/vmdk:
Report failures in vmdk_read_cid()') and needs to be dealt with
separately.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
