[build] Drag in Xen and Hyper-V support via network device drivers

Include Xen and Hyper-V support in the all-drivers build by dragging
in the netfront and netvsc drivers, since these are the functional
drivers that provide network interfaces.

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

src/interface/xen/xenbus.c

@@ -400,3 +400,9 @@ void xenbus_remove ( struct xen_hypervisor *xen __unused,
 		xenbus_remove_device ( xendev );
 	}
 }
+
+/* Drag in objects via xenbus_set_state() */
+REQUIRING_SYMBOL ( xenbus_set_state );
+
+/* Drag in backing hardware device */
+REQUIRE_OBJECT ( hvm );