etc/ttys: add xen console

Xen VMs get a simulated serial device meant for use as a console. Often
an xterm or other advanced terminal is used, so use xterm as the type.

Depending on configuration, FreeBSD on Xen for amd64 may instead use an
emulated serial port, but the virtual console may also be available.

Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: imp (slightly earlier version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29873