xci: scripts: start-new-vm.sh: Do not destroy user's configuration file

xci: scripts: start-new-vm.sh: Do not destroy user's configuration file

As we move towards making this script useful for developers, we
shouldn't destroy the existing ssh configuration file but rather
create a new one just for XCI. This also move the destruction of
previous instances sooner, even before building the OS image so we
can free resources as soon as possible. Finally, this also uses the
hostname to ssh to the new VM instead of its IP to ensure that
everything is setup up properly on the physical host.

Change-Id: I185befd8943fdb8ebcdb635aefb4d4b03473258b
Signed-off-by: Markos Chandras <mchandras@suse.de>