selftests/hid: vmtest.sh: allow finer control on the build steps

vmtest.sh works great for a one shot test, but not so much for CI where
I want to build (with different configs) the bzImage in a separate
job than the one I am running it.

Add a "build_only" option to specify whether we need to boot the currently
built kernel in the vm.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Jiri Kosina <jkosina@suse.com>
Link: https://lore.kernel.org/r/20231206-wip-selftests-v2-2-c0350c2f5986@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>