OvmfPkg/XenBusDxe: Don't call DisconnectController in Stop()

OvmfPkg/XenBusDxe: Don't call DisconnectController in Stop()

Calling DisconnectController() on children isn't part of the job of
EFI_DRIVER_BINDING_PROTOCOL.Stop() as it only needs to deallocate
resources allocated in Start(). The disconnection will happen when
both DevicePath and XenBus protocols gets uninstalled.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190701111403.7007-1-anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>