char: introduce support for TLS encrypted TCP chardev backend

This integrates support for QIOChannelTLS object in the TCP
chardev backend. If the 'tls-creds=NAME' option is passed with
the '-chardev tcp' argument, then it will setup the chardev
such that the client is required to establish a TLS handshake
when connecting. There is no support for checking the client
certificate against ACLs in this initial patch. This is pending
work to QOM-ify the ACL object code.

A complete invocation to run QEMU as the server for a TLS
encrypted serial dev might be

  $ qemu-system-x86_64 \
      -nodefconfig -nodefaults -device sga -display none \
      -chardev socket,id=s0,host=127.0.0.1,port=9000,tls-creds=tls0,server \
      -device isa-serial,chardev=s0 \
      -object tls-creds-x509,id=tls0,endpoint=server,verify-peer=off,\
         dir=/home/berrange/security/qemutls

To test with the gnutls-cli tool as the client:

  $ gnutls-cli --priority=NORMAL -p 9000 \
       --x509cafile=/home/berrange/security/qemutls/ca-cert.pem \
       127.0.0.1

If QEMU was told to use 'anon' credential type, then use the
priority string 'NORMAL:+ANON-DH' with gnutls-cli

Alternatively, if setting up a chardev to operate as a client,
then the TLS credentials registered must be for the client
endpoint. First a TLS server must be setup, which can be done
with the gnutls-serv tool

  $ gnutls-serv --priority=NORMAL -p 9000 --echo \
       --x509cafile=/home/berrange/security/qemutls/ca-cert.pem \
       --x509certfile=/home/berrange/security/qemutls/server-cert.pem \
       --x509keyfile=/home/berrange/security/qemutls/server-key.pem

Then QEMU can connect with

  $ qemu-system-x86_64 \
      -nodefconfig -nodefaults -device sga -display none \
      -chardev socket,id=s0,host=127.0.0.1,port=9000,tls-creds=tls0 \
      -device isa-serial,chardev=s0 \
      -object tls-creds-x509,id=tls0,endpoint=client,\
        dir=/home/berrange/security/qemutls

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1453202071-10289-5-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
