tests: Optionally start telnet server inside VMs

tests: Optionally start telnet server inside VMs

If telnetd is installed and --telnet <port> is passed on the
vm-run.sh command line, start a telnet server (directly connected
to bash, no login) inside the VM(s) to be able to look into them
when something is wrong. Use a user network in qemu with a single
host forward from the specified port for this, listening only on
'localhost'.

Please note that this provides unauthenticated access to the guest
system from anything that can open a TCP connection on the host system.
The guess system does have access to reading all files on the host that
the user account running kvm has access to (and even write access if the
default ROTAG ,readonly parameter is cleared). In other words, this
option should not be used on any multiuser systems where kvm is run
under user accounts that are not dedicated for testing purposes (i.e.,
do not have access to any files that should not be readable to
everyone).

This needs CONFIG_VIRTIO_NET=y in the guest kernel.

For parallel-vm.py, the --telnet argument specifies the base port
and each VM index (0, 1, ...) is added to it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>