migration: use "" as the default for tls-creds/hostname

The tls-creds parameter has a default value of NULL indicating
that TLS should not be used. Setting it to non-NULL enables
use of TLS. Once tls-creds are set to a non-NULL value via the
monitor, it isn't possible to set them back to NULL again, due
to current implementation limitations. The empty string is not
a valid QObject identifier, so this switches to use "" as the
default, indicating that TLS will not be used

The tls-hostname parameter has a default value of NULL indicating
the the hostname from the migrate connection URI should be used.
Again, once tls-hostname is set non-NULL, to override the default
hostname for x509 cert validation, it isn't possible to reset it
back to NULL via the monitor. The empty string is not a valid
hostname, so this switches to use "" as the default, indicating
that the migrate URI hostname should be used.

Using "" as the default for both, also means that the monitor
commands "info migrate_parameters" / "query-migrate-parameters"
will report existance of tls-creds/tls-parameters even when set
to their default values.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

Signed-off-by: Juan Quintela <quintela@redhat.com>