vmnetx.ui.view: Fix missing guest mouse cursor in some sessions

We can't use channel.connect() to connect a GObject signal because
SpiceChannel overrides connect() to mean "initiate a TCP connection".
Instead, we were using channel.connect_object(..., channel), but this
creates a reference cycle between the channel PyGObject and the
underlying GObject.  With connect(), the PyGObject would be destroyed
after _new_channel() and recreated (pointing to the same GObject) before
the _request_fd() callback.  With connect_object(), the PyGObject is
referenced from the signal handler closure and so is never freed.
However, if the Python GC runs between _new_channel() and _request_fd(),
it will detect a reference cycle and clear the PyGObject's reference to
the GObject.  This was happening to the cursor channel on RHEL 6;
_request_fd() would receive an invalid PyGObject and then swallow the
resulting TypeError.  As a result, the cursor channel would never be
connected, and the guest cursor (for VMs supporting a hardware cursor)
would always be invisible.

Use connect() instead of connect_object(), explicitly calling it through
the superclass.