tests/iotests: drop test 312 from auto group

tests/iotests: drop test 312 from auto group

The "auto" documentation states:

  That means they should run with every QEMU binary (also non-x86)

which is not the case as the check-system-fedora build which only
includes a rag tag group of rare and deprecated targets doesn't
support the virtio device required.

Fixes: ef9bba1484b ("quorum: Implement bdrv_co_block_status()")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210105100402.12350-1-alex.bennee@linaro.org>