conf: prevent crash with no uuid in cephx auth secret

conf: prevent crash with no uuid in cephx auth secret

Fix the null pointer access when UUID is not specified.
Introduce a bool 'uuidUsable' to virStoragePoolAuthCephx that indicates
if uuid was specified or not and use it instead of the pointless
comparison of the static UUID array to NULL.
Add an error message if both uuid and usage are specified.

Fixes:
Error: FORWARD_NULL (CWE-476):
libvirt-0.10.2/src/conf/storage_conf.c:461: var_deref_model: Passing
    null pointer "uuid" to function "virUUIDParse(char const *, unsigned
    char *)", which dereferences it. (The dereference is assumed on the
    basis of the 'nonnull' parameter attribute.)
Error: NO_EFFECT (CWE-398):
    libvirt-0.10.2/src/conf/storage_conf.c:979: array_null: Comparing an
    array to null is not useful: "src->auth.cephx.secret.uuid != NULL".