libxl_conf: Fix config generation for multiple serial devices

libxl_conf: Fix config generation for multiple serial devices

Currently, an array of libxl_string_list (char **) or in other words,
a triple char pointer is initialized. This is dereferenced to a char ** type
and stored in serial_list, which is NULL at this point. There is an attempt to
reference an element of this serial_list when making a call to
libxlMakeChrdevStr which causes a segmentation fault.

To fix this, we simply allocate an array of char * instead of
libxl_string_list.

This patch also adds testcases to extend coverage over both single serial and
multiple serial cases.

Signed-off-by: Rayhan Faizel <rayhan.faizel@gmail.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>