So far, none of the existing callers of vshStringToArray expected
the user to ever pass a literal comma; meanwhile, snapshot parsing
had rolled its own array parser.  Moving the comma escaping into
the common function won't affect any existing callers, and will make
this function reusable for adding memory handling to snapshot parsing.

As a bonus, the testsuite was already testing snapshot parsing, so
the fact that the test still passes means that we are now giving
testsuite exposure to vshStringToArray.

* tools/virsh-snapshot.c (vshParseSnapshotDiskspec): Move ,,
parsing...
* tools/virsh.c (vshStringToArray): ...into common function.
Also, vshStrdup can't fail.