[efi] Dump existing openers when we are unable to open a protocol

Dump the existing openers of a protocol whenever we are unable to open
a protocol using attributes of BY_DEVICE, EXCLUSIVE, or
BY_CHILD_CONTROLLER.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/efi/snpnet.c

@@ -407,6 +407,8 @@ int snpnet_start ( struct efi_device *efidev ) {
 		rc = -EEFI ( efirc );
 		DBGC ( device, "SNP %p %s cannot open SNP protocol: %s\n",
 		       device, efi_handle_name ( device ), strerror ( rc ) );
+		DBGC_EFI_OPENERS ( device, device,
+				   &efi_simple_network_protocol_guid );
 		goto err_open_protocol;
 	}