crypto: add support for loading encrypted x509 keys

Make use of the QCryptoSecret object to support loading of
encrypted x509 keys. The optional 'passwordid' parameter
to the tls-creds-x509 object type, provides the ID of a
secret object instance that holds the decryption password
for the PEM file.

 # printf "123456" > mypasswd.txt
 # $QEMU \
    -object secret,id=sec0,filename=mypasswd.txt \
    -object tls-creds-x509,passwordid=sec0,id=creds0,\
            dir=/home/berrange/.pki/qemu,endpoint=server \
    -vnc :1,tls-creds=creds0

This requires QEMU to be linked to GNUTLS >= 3.1.11. If
GNUTLS is too old an error will be reported if an attempt
is made to pass a decryption password.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>