vmgenid-test: use boot-sector infrastructure

There's no requirement for RSDP to be installed last
by the firmware, so in rare cases vmgen id test hits
a race: RSDP is there but VM GEN ID isn't.

To fix, switch to common boot sector infrastructure.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Ben Warren <ben@skyportsystems.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Ben Warren <ben@skyportsystems.com>
Message-id: 1500046217-24597-1-git-send-email-mst@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>