mpath: Don't allow more than one mpath pool at a time

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).