tests/vm: freebsd autoinstall, using serial console

tests/vm: freebsd autoinstall, using serial console

Instead of fetching the prebuilt image from patchew download the install
iso and prepare the image locally.  Install to disk, using the serial
console.  Create qemu user, configure ssh login.  Install packages
needed for qemu builds.

Note that freebsd package downloads are delivered as non-cachable
content, so I had to configure squid with "ignore-no-store
ignore-private ignore-reload" for pkgmir.geo.freebsd.org to make the
caching actually work.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190617043858.8290-9-kraxel@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>