mpath: Don't allow more than one mpath pool at a time
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1232606
    
    Since an mpath pool contains all the Multipath devices on a host, allowing
    more than one defined on a host at a time should be disallowed under the
    policy of disallowing duplicate source pools for the host.
    
    Adjust to docs to clarify the Multipath target path value usage for both
    the storage driver (only 1 pool per host) and formatstorage references
    (ignore the target element in favor of the default target mapping of
    /dev/mapper).