libxlu_cfg: reject unknown characters following '\'

libxlu_cfg: reject unknown characters following '\'

When dequoting config strings in xlu__cfgl_dequote(), unknown
characters following a '\', and the '\' itself, are discarded.
E.g. a disk configuration string containing

  rbd:pool/image:mon_host=192.168.0.100\:6789

would be dequoted as

  rbd:pool/image:mon_host=192.168.0.1006789

Instead of discarding the '\' and unknown character, reject the
string and set error to EINVAL.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>