qemu: migration: Set the 'set' boolean in qemuMigrationParamsSetString
    
    The code setting TLS parameters verifies that TLS is supported by
    looking at the dump of parameters which will be reset after migration,
    but sets the parameters in the list of new parameters. As
    qemuMigrationParamsSetString did not set the 'set' property, the TLS
    parameters would not be used.
    
    This is a regression after the series refactoring migration parameters
    and it resulted into TLS not being used even when requested.
    
    Signed-off-by: Peter Krempa <pkrempa@redhat.com>