usb-acl-helper: Avoid deadlock when cancelled
usb-acl-helper: Avoid deadlock when cancelled

cancelled_cb() (which is triggered when the GCancellable's "cancelled"
signal is emitted) called spice_usb_acl_helper_close_acl(), which calls
spice_usb_acl_helper_cleanup(), which in turn calls
g_cancellable_disconnect(). Calling g_cancellable_disconnect() from
within a "cancelled" handler results in a dealock, as mentioned in the
documentation. Instead of closing the acl here, simply cancel the task
here. The cleanup() call will happen when the SpiceUsbAclHelper object
is destroyed.

SPICE/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk