tests/vm: update openbsd image to 7.4

tests/vm: update openbsd image to 7.4

The old links are dead so even if we have the ISO cached we can't
finish the install. Update to the current stable and tweak the install
strings.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2192
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-5-alex.bennee@linaro.org>
(cherry picked from commit 8467ac75b3b7207a49a1c6c7b87f0f7d2d0cea18)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>