multi-boot for openstack/qcow images

Make stock FreeBSD more useful for people wishing to use them. The
QEMU folks suggested this change. It adds a serial console which
allows them to interact with FreeBSD from the earliest moments. This
allows them to configure FreeBSD via the serial port to set it up for
CI use.

Reviewed by: kevans@
Sponsored by: Netflix, Inc
Differential Revision: https://reviews.freebsd.org/D22786