tests: boot_sector_test(): make it multi-shot

tests: boot_sector_test(): make it multi-shot

if the function is called the 2nd time within the same qtest session,
it will prematurely return before boot sector is executed due to
remaining signature.

Follow up patch will add VM reboot to a test case and will
call boot_sector_test() again within the same qtest env,
which may lead to above issue.

To fix it make sure signature in VM RAM is no more before
exiting boot_sector_test(), so next time it's called it
will wait boot sector is completed again.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20230112140312.3096331-16-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>