xen-legacy-backend: Add missing typedef XenLegacyDevice

The typedef was used in the XENBACKEND_DEVICE macro, but it was
never defined.  Define the typedef close to the type checking
macro.

Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-By: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200825192110.3528606-27-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

include/hw/xen/xen-legacy-backend.h

@@ -9,6 +9,7 @@
 #define TYPE_XENSYSBUS "xen-sysbus"
 #define TYPE_XENBACKEND "xen-backend"
 
+typedef struct XenLegacyDevice XenLegacyDevice;
 #define XENBACKEND_DEVICE(obj) \
     OBJECT_CHECK(XenLegacyDevice, (obj), TYPE_XENBACKEND)