fw_cfg: switch fw_cfg_find() to locate the fw_cfg device by type rather than path

fw_cfg: switch fw_cfg_find() to locate the fw_cfg device by type rather than path

This will enable the fw_cfg device to be placed anywhere within the QOM tree
regardless of its machine location.

Note that we also add a comment to document the behaviour that we return NULL to
indicate failure where either no fw_cfg device or multiple fw_cfg devices are
found.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <1500025208-14827-2-git-send-email-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>