virsh: Improve checking for connection when running commands

virsh: Improve checking for connection when running commands

Almost each virsh command uses the function vshConnectionUsability
before doing anything, to check if the connection is "alive".  Commands
that don't need an conection are already conveniently marked with
VSH_CMD_FLAG_NOCONNECT. We can automaticaly check for the connection
before calling any remote command so we don't forget to do so.

This patch also upgrades the connection check to use virConnectIsAlive
along with the current approach.