virsh: Don't infloop on snapshot/storage_vol failure

virsh: Don't infloop on snapshot/storage_vol failure

Most of our completers used the pattern:
if ((nITEM = virITEMListAll()) < 0)
    return NULL;

but the virDomainSnapshot and virStorageVolume completers were instead
using goto error. If the ListAll fails with -1, the cleanup label was
running a loop of 'size_t i < int nITEM', which is an extreme waste of
CPU cycles. Broken since their introduction in v4.1.

Fixes: f81f8b62
Fixes: 4cb4b649
Reported-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>