add --description to systemd scope unit

add --description to systemd scope unit

otherwise, long kvm commands lead to systemd unit files with
very long lines, with confuses the systemd unit file parser.

apparently systemd has a length limit for unit file lines and
(line-)breaks the description string at that point. since
the rest of the description is probably not a valid key/value
pair, this leads to warnings. the default semantics of systemd-run
is to use the executed command as description unless a description
is specified explicitly.

note that this behaviour of systemd could allow an attacker
with access to the VM configuration to craft a kvm commandline
that starts or stops arbitrary systemd units.