Add helper APIs for generating cryptographic hashes

GNULIB provides APIs for calculating md5 and sha256 hashes,
but these APIs only return you raw byte arrays. Most users
in libvirt want the hash in printable string format. Add
some helper APIs in util/vircrypto.{c,h} for doing this.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

include/libvirt/virterror.h

@@ -121,6 +121,7 @@ typedef enum {
     VIR_FROM_ACCESS = 55,       /* Error from access control manager */
     VIR_FROM_SYSTEMD = 56,      /* Error from systemd code */
     VIR_FROM_BHYVE = 57,        /* Error from bhyve driver */
+    VIR_FROM_CRYPTO = 58,       /* Error from crypto code */
 
 # ifdef VIR_ENUM_SENTINELS
     VIR_ERR_DOMAIN_LAST