migration: add support for encrypting data with TLS

This extends the migration_set_incoming_channel and
migration_set_outgoing_channel methods so that they
will automatically wrap the QIOChannel in a
QIOChannelTLS instance if TLS credentials are configured
in the migration parameters.

This allows TLS to work for tcp, unix, fd and exec
migration protocols. It does not (currently) work for
RDMA since it does not use these APIs, but it is
unlikely that TLS would be desired with RDMA anyway
since it would degrade the performance to that seen
with TCP defeating the purpose of using RDMA.

On the target host, QEMU would be launched with a set
of TLS credentials for a server endpoint

 $ qemu-system-x86_64 -monitor stdio -incoming defer \
    -object tls-creds-x509,dir=/home/berrange/security/qemutls,endpoint=server,id=tls0 \
    ...other args...

To enable incoming TLS migration 2 monitor commands are
then used

  (qemu) migrate_set_str_parameter tls-creds tls0
  (qemu) migrate_incoming tcp:myhostname:9000

On the source host, QEMU is launched in a similar
manner but using client endpoint credentials

 $ qemu-system-x86_64 -monitor stdio \
    -object tls-creds-x509,dir=/home/berrange/security/qemutls,endpoint=client,id=tls0 \
    ...other args...

To enable outgoing TLS migration 2 monitor commands are
then used

  (qemu) migrate_set_str_parameter tls-creds tls0
  (qemu) migrate tcp:otherhostname:9000

Thanks to earlier improvements to error reporting,
TLS errors can be seen 'info migrate' when doing a
detached migration. For example:

  (qemu) info migrate
  capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: off compress: off events: off x-postcopy-ram: off
  Migration status: failed
  total time: 0 milliseconds
  error description: TLS handshake failed: The TLS connection was non-properly terminated.

Or

  (qemu) info migrate
  capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: off compress: off events: off x-postcopy-ram: off
  Migration status: failed
  total time: 0 milliseconds
  error description: Certificate does not match the hostname localhost

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-27-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>