qemu-iotests: fix 017 018 for vmdk

qemu-iotests: fix 017 018 for vmdk

017 and 018 use /bin/mv to move base img from t.IMGFMG to t.IMGFMT.base
after filling data, this is not enough for vmdk, when t.IMGFMT is only a
description text file who points to t-{flat,s001,f001,...}.IMGFMT as
data extent, so testing such subformats alway fails on them.

This patch use the trick of temprorily changing TEST_IMG to avoid using
/bin/mv.

Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>