xen: unmask event channels on listen and connect
xen: unmask event channels on listen and connect

The new mini-os is careful to create the channels masked.
We must therefore unmask them before we can use them.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

mirage/ocaml-vchan,samoht/ocaml-vchan,samoht/ocaml-vchan,djs55/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan,samoht/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan