lib: Introduce and use g_autoptr() for virInterfaceDef

lib: Introduce and use g_autoptr() for virInterfaceDef

There are a lot of places where we call virInterfaceDefFree()
explicitly. We can define autoptr cleanup macro and annotate
declarations with g_autoptr() and remove plenty of those explicit
free calls.

This also fixes a memory leak in udevInterfaceGetXMLDesc() which
called virInterfaceDefFree() only in successful path.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>