block: convert quorum blockdrv to use crypto APIs

Get rid of direct use of gnutls APIs in quorum blockdrv in
favour of using the crypto APIs. This avoids the need to
do conditional compilation of the quorum driver. It can
simply report an error at file open file instead if the
required hash algorithm isn't supported by QEMU.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1435770638-25715-8-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>