xen: use a common function for pv and hvm guest backend register calls

Instead of calling xen_be_register() for each supported backend type
for hvm and pv guests in their machine init functions use a common
function in order not to have to add new backends twice.

This at once fixes the error that hvm domains couldn't use the qusb
backend.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Message-id: 1470119552-16170-1-git-send-email-jgross@suse.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

xen-hvm.c

@@ -1318,9 +1318,7 @@ void xen_hvm_init(PCMachineState *pcms, MemoryRegion **ram_memory)
         error_report("xen backend core setup failed");
         goto err;
     }
-    xen_be_register("console", &xen_console_ops);
-    xen_be_register("vkbd", &xen_kbdmouse_ops);
-    xen_be_register("qdisk", &xen_blkdev_ops);
+    xen_be_register_common();
     xen_read_physmap(state);
     return;