conf: interleave virstoragetest structs

conf: interleave virstoragetest structs

As I add more tests, it's getting harder to follow the split between
a struct in one place and a test using the struct in another.
Interleaving the tests makes changes more localized, and also makes
debugging easier when a test goes wrong during my refactoring work.

* tests/virstoragetest.c (mymain): Modify structs as we go, rather
than up-front.
(testStorageChain): Make failure debugging easier.

Signed-off-by: Eric Blake <eblake@redhat.com>