virstoragetest: Stop rewriting images in 'mymain'

virstoragetest: Stop rewriting images in 'mymain'

For testing of real images formatted by 'qemu-img' it's now sufficient
to format them once without the need to rewrtie them since we use the
real images only for testing of one scenario.

This allows us to also remove most of the global variables holding the
path to the images which was necessary when they were being rewritten.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>