iotests: Improve and rename test 309 to nbd-qemu-allocation

Enhance the test to inspect what qemu-nbd is advertising during
handshake, and rename it now that we support useful iotest names.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210701190655.2131223-2-eblake@redhat.com>
2 files changed