migration: define 'tls-creds' and 'tls-hostname' migration parameters

Define two new migration parameters to be used with TLS encryption.
The 'tls-creds' parameter provides the ID of an instance of the
'tls-creds' object type, or rather a subclass such as 'tls-creds-x509'.
Providing these credentials will enable use of TLS on the migration
data stream.

If using x509 certificates, together with a migration URI that does
not include a hostname, the 'tls-hostname' parameter provides the
hostname to use when verifying the server's x509 certificate. This
allows TLS to be used in combination with fd: and exec: protocols
where a TCP connection is established by a 3rd party outside of
QEMU.

NB, this requires changing the migrate_set_parameter method in the
HMP to accept a 's' (string) value instead of 'i' (integer). This
is backwards compatible, because the parsing of strings allows the
quotes to be optional, thus any integer is also a valid string.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-26-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
