virt: Register "its" as class property

virt: Register "its" as class property

Class properties make QOM introspection simpler and easier, as
they don't require an object to be instantiated.

Note: "its" is currently registered conditionally, but this makes
the feature be registered unconditionally.  The only side effect
is that it will be now possible to set its=on on virt-2.7 and
older.

Signed-off-by: Eduardo Habkost <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-Id: <20201111183823[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>