crypto: introduce new module for computing hash digests

Introduce a new crypto/ directory that will (eventually) contain
all the cryptographic related code. This initially defines a
wrapper for initializing gnutls and for computing hashes with
gnutls. The former ensures that gnutls is guaranteed to be
initialized exactly once in QEMU regardless of CLI args. The
block quorum code currently fails to initialize gnutls so it
only works by luck, if VNC server TLS is not requested. The
hash APIs avoids the need to litter the rest of the code with
preprocessor checks and simplifies callers by allocating the
correct amount of memory for the requested hash.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1435770638-25715-2-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>