tests/boot-sector: Drop dependence on global_qtest

As a general rule, we prefer avoiding implicit global state
because it makes code harder to safely copy and paste without
thinking about the global state.  Adjust the helper code to
use explicit state instead, and update all callers.

Fix some trailing whitespace while touching the file.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>