libxlu: Handle += in config files

libxlu: Handle += in config files

Handle += of both strings and lists.

If += is used for config options expected to be numbers, then a
warning is printed and the config option ignored (because xl ignores
config options with errors).

This is to be used for development purposes, where modifying config
option can be done on the `xl create' command line.

One could have a cmdline= in the cfg file, and specify cmdline+= on
the `xl create` command line without the need to write the whole
cmdline in `xl' command line but simply append to the one in the cfg file.
Or add an extra vif or disk by simply having "vif += [ '', ];" in the
`xl' cmdline.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>