monitor: Fix memory leak with readline completion

Each string which is shown during readline completion in the QEMU monitor
is allocated dynamically but currently never deallocated.

Add the missing loop which calls g_free for the allocated strings.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
