virsysinfo: Define and use auto cleanup func for virSysinfoDef properly

virsysinfo: Define and use auto cleanup func for virSysinfoDef properly

What we are using really is heap allocated structure rather than
stack allocated. And for that it's better to use g_autoptr() +
G_DEFINE_AUTOPTR_CLEANUP_FUNC() combo, as Glib documentation for
g_auto() reads:

  This is meant to be used with stack-allocated structures and
  non-pointer types. For the (more commonly used) pointer
  version, see g_autoptr().

This will be even more visible, when virSysinfoDefPtr type is
gone. Stay tuned.

Fixes: cee3a900a0d6a8fc79554db22dc262632fe487a6
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>