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>