virstoragetest: Don't skip the whole test when qemu-img fails to format images
virstoragetest: Don't skip the whole test when qemu-img fails to format images

We have plenty of other work to do in this test. Skip only the real
image testing case when we can't find qemu-img or it failed to format
the image.

This allows us to also remove the last global variable in the test and
move the creation and cleanup of the images closer to the actual test.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,zippy2/libvirt