qom: Change object_get_canonical_path_component() not to malloc

qom: Change object_get_canonical_path_component() not to malloc

object_get_canonical_path_component() returns a malloced copy of a
property name on success, null on failure.

19 of its 25 callers immediately free the returned copy.

Change object_get_canonical_path_component() to return the property
name directly.  Since modifying the name would be wrong, adjust the
return type to const char *.

Drop the free from the 19 callers become simpler, add the g_strdup()
to the other six.

Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20200714160202.3121879[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Li Qiang <[email protected]>