client.c: Created read_xml_file() to read the XML data file

read_xml_file() creates xmlInventory string and copies the XML data file,
returning a pointer to it in success or NULL in case of error.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Diego Giovane Pasqualin <dgp06@c3sl.ufpr.br>
Acked-by: Josiney de Souza <josineys@c3sl.ufpr.br>
Acked-by: Ricardo Tavares de Oliveira <rto07@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>