qga: Use return values instead of error_is_set(errp)

Using error_is_set(errp) to check whether a function call failed is
fragile: it breaks when errp is null.  ga_get_fd_handle() and
guest_file_handle_add() don't return a useful value when they fail,
but that's just stupid.  Fix that, and check them instead.  As far
as I can tell, errp can't be null there, but this is more robust and
more obviously correct.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qga/main.c

@@ -910,6 +910,7 @@ int64_t ga_get_fd_handle(GAState *s, Error **errp)
 
     if (!write_persistent_state(&s->pstate, s->pstate_filepath)) {
         error_setg(errp, "failed to commit persistent state to disk");
+        return -1;
     }
 
     return handle;