virsh: Remember terminal state when starting and add helpers

virsh: Remember terminal state when starting and add helpers

This patch adds instrumentation to allow modification of config of the
terminal in virsh and successful reset of the state afterwards.

The added helpers allow to disable receiving of SIGINT when pressing the
key sequence (Ctrl+C usualy). This normally sends SIGINT to the
foreground process group which kills ssh processes used for transport of
the data.