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: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>