util: add virTrimSpaces

util: add virTrimSpaces

The next patch wants to adjust an end pointer to trim trailing
spaces but without modifying the underlying string, but a more
generally useful ability to trim trailing spaces in place is
also worth providing.

* src/util/util.h (virTrimSpaces, virSkipSpacesBackwards): New
prototypes.
* src/util/util.c (virTrimSpaces, virSkipSpacesBackwards): New
functions.
* src/libvirt_private.syms (util.h): Export new functions.
Inspired by a patch by Minoru Usui.