libmultipath: use vector for for pp->hwe and mp->hwe

libmultipath: use vector for for pp->hwe and mp->hwe

Change the data structure of "struct path" and "struct multipath"
such that the "hwe" entry is a vector of hwentry structures rather
than a single hwentry structure. Add respective code to the
constructors and destructors (note that mp->hwe is never allocated,
it's always a pointer to a path hwe).

Change find_hwe() to fill in the passed vector rather than returning
a hwentry pointer. Change the propsel code to look through vectors
of hwentries to determine a given property.

This patch just creates the new data structure and the functions to
deal with them, it doesn't introduce semantic changes.

Signed-off-by: Martin Wilck <mwilck@suse.com>