migration: Use JSON null instead of "" to reset parameter to default

migrate-set-parameters sets migration parameters according to is
arguments like this:

* Present means "set the parameter to this value"

* Absent means "leave the parameter unchanged"

* Except for parameters tls_creds and tls_hostname, "" means "reset
  the parameter to its default value

The first two are perfectly normal: presence of the parameter makes
the command do something.

The third one overloads the parameter with a second meaning.  The
overloading is *implicit*, i.e. it's not visible in the types.  Works
here, because "" is neither a valid TLS credentials ID, nor a valid
host name.

Pressing argument values the schema accepts, but are semantically
invalid, into service to mean "reset to default" is not general, as
suitable invalid values need not exist.  I also find it ugly.

To clean this up, we could add a separate flag argument to ask for
"reset to default", or add a distinct value to @tls_creds and
@tls_hostname.  This commit implements the latter: add JSON null to
the values of @tls_creds and @tls_hostname, deprecate "".

Because we're so close to the 2.10 freeze, implement it in the
stupidest way possible: have qmp_migrate_set_parameters() rewrite null
to "" before anything else can see the null.  The proper way to do it
would be rewriting "" to null, but that requires fixing up code to
work with null.  Add TODO comments for that.

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