libusbhost: permits client polling on inotify wd

Modify libusbhost to expose the inotify watch descriptor to clients

This modification permits clients to add the watch descriptor to
their polling loop so that they don't have to use a dedicated
thread only for libusbhost.

Change-Id: I615bfcd56beab978135034b228d4d93337351eab
Signed-off-by: Guillaume Ranquet <guillaumex.ranquet@intel.com>
Signed-off-by: Luc Piguet-Lacroix <lucx.piguet-lacroix@intel.com>