xen: Fix IDE unplug

After commit e5e78550 (blockdev: Separate BB name management), starting a
guest with PVHVM support result in this assert:
qemu-system-i386: block/block-backend.c:173: blk_delete: Assertion `!blk->name' failed.

A backtrace show that a caller is pci_piix3_xen_ide_unplug().

This patch fix it.
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Message-id: 1460382666-29885-1-git-send-email-anthony.perard@citrix.com
Signed-off-by: NJohn Snow <jsnow@redhat.com>

hw/ide/piix.c

@@ -189,6 +189,7 @@ int pci_piix3_xen_ide_unplug(DeviceState *dev)
                 idedev = pci_ide->bus[di->bus].slave;
             }
             idedev->conf.blk = NULL;
+            monitor_remove_blk(blk);
             blk_unref(blk);
         }
     }