tests/vm: avoid re-building the VM images all the time

tests/vm: avoid re-building the VM images all the time

The main problem is that "check-venv" is a .PHONY target will always
evaluate and trigger a full re-build of the VM images. While its
tempting to drop it from the dependencies that does introduce a
breakage on freshly configured builds.

Fortunately we do have the otherwise redundant --force flag for the
script which up until now was always on. If we make the usage of
--force conditional on dependencies other than check-venv triggering
the update we can avoid the costly rebuild and still run cleanly on a
fresh checkout.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2118
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-4-alex.bennee@linaro.org>