block: don't register quorum driver if SHA256 support is unavailable

Commit 488981a4 [block: convert quorum blockdrv to use crypto APIs]
broke qemu-iotest 041 on hosts with GnuTLS < 2.10.0. It converted a
compile-time check to a run-time check at device open time. The result
is that we now advertise a feature (the quorum block driver) that will
never work (on those hosts). There's no way (short of parsing
human-readable error messages) for qemu-iotests or any other API
consumer to recognise that the quorum block driver isn't _actually_
available and shouldn't be used or tested.

Move the run-time check to bdrv_quorum_init() to avoid registering the
quorum block driver if we know it cannot work. This way API consumers
can recognise it's unavailable.

Fixes: 488981a4af396551a3178d032cc2b41d9553ada2
Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1438699705-21761-1-git-send-email-silbe@linux.vnet.ibm.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
