fw_cfg: add API to find FW cfg object

Remove some code duplication by adding a
function to look up the fw cfg file.
This way, we don't need to duplicate same strings everywhere.
Use by both fw cfg and pvpanic device.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/nvram/fw_cfg.h

@@ -73,6 +73,8 @@ void fw_cfg_add_file(FWCfgState *s, const char *filename, void *data,
 FWCfgState *fw_cfg_init(uint32_t ctl_port, uint32_t data_port,
                         hwaddr crl_addr, hwaddr data_addr);
 
+FWCfgState *fw_cfg_find(void);
+
 #endif /* NO_QEMU_PROTOS */
 
 #endif