xen-netback: transition to CLOSED when removing a VIF

xen-netback: transition to CLOSED when removing a VIF

If a guest is destroyed without transitioning its frontend to CLOSED,
the domain becomes a zombie as netback was not grant unmapping the
shared rings.

When removing a VIF, transition the backend to CLOSED so the VIF is
disconnected if necessary (which will unmap the shared rings etc).

This fixes a regression introduced by
279f438e36c0a70b23b86d2090aeec50155034a9 (xen-netback: Don't destroy
the netdev until the vif is shut down).

Signed-off-by: David Vrabel <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Wei Liu <[email protected]>
Cc: Paul Durrant <[email protected]>
Acked-by: Wei Liu <[email protected]>
Reviewed-by:  Paul Durrant <[email protected]>
Signed-off-by: David S. Miller <[email protected]>