xen-netback: Handle backend state transitions in a more robust way

commit ea732dff upstream.

When the frontend state changes netback now specifies its desired state to
a new function, set_backend_state(), which transitions through any
necessary intermediate states.
This fixes an issue observed with some old Windows frontend drivers where
they failed to transition through the Closing state and netback would not
behave correctly.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ kamal: backport to 3.8 (context); prereq for
  dc62ccac "xen-netback: transition to CLOSED when removing a VIF" ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>