libxl, xl: don't free string literals in config structure

libxl, xl: don't free string literals in config structure

The function init_dm_info() is initialising some strings from literals.
This is bad juju because when the destructor is called we cannot know if
the string literal was overridden with a strdup()'d value. Therefore
strdup values in the initialiser then introduce and use the function
libxlu_cfg_replace_string() which free's whatever is set before
strdupping the new value on top of it. The rule for the new call should
be clear due to const vs. non-const arguments - changing the behaviour
of libxlu_cfg_get_string() would cause more complexity than it saves.

[ fixed up for stray ! sign -iwj ]

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>