xen_common: Change xenConfigGetString to use virConfGetValueString

xen_common: Change xenConfigGetString to use virConfGetValueString

This change actually changes the behaviour of xenConfigGetString() as
now it returns a newly-allocated string.

Unfortunately, there's not much that can be done in order to avoid that
and all the callers have to be changed in order to avoid leaking the
return value.

Also, as a side-effect of the change above, the function now takes a
"char **" argument instead of a "const char **" one.

Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>