boot_order_source_priority fails with Debian 11 guest

[iximeow]

found while digging into #781: repro steps are basically identical. get a Debian 11 image, get Propolis from any commit at or after 11371b0.

run the test with an appropriate adaptation of a command like:

./target/debug/phd-runner run \
    --propolis-server-cmd ./target/release/propolis-server \
    --tmp-directory /tmp/phd \
    --artifact-toml-path ./artifacts-debian-bullseye.toml \
    --default-guest-artifact debian_bullseye \
    --include-filter boot_order_source_priority

this should exit with something like test result: ok. 1 passed; 0 failed; 0 skipped; 0 not run; finished in 23.39s - this is the result with an Alpine guest.

instead, with a Debian 11 guest, this hangs at Waiting for serial output on serial console target="root@debian:~#" and eventually times out.

the tell here is that the line just before is Waiting for output on serial console target="reboot". that is from this reboot, where we're waiting for the new line after sending reboot. it turns out on Debian 11, reboot just reboots the machine immediately, but on more recent linuxes (recent Alpine, Ubuntu 22.04, etc) this is just a compatibility adapter for, it seems like, some systemd incantation that will reboot the machine in the near future.

this is why "reboot and wait for login" works as a strategy for more recent Linux images, but just hangs on Debian 11. on Debian 11, we even cycle all the way through to the login prompt, it's just not what we expected to see in response to reboot, so we keep waiting.

i figure i'll add a helper in the guest adaptations for "Perform graceful reboot initiated by the guest", which should fix this in the process.