[libmultipath] Fix debugging output

[libmultipath] Fix debugging output

Implement a proper output function for libdevmapper which ties
into our verbosity level. This allows us for a finer grained
control about the error messages from libdevmapper and we get
better debugging output to boot.

Signed-off-by: Stefan Bader <bader@de.ibm.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>