mmio-interface: Mark as not user creatable

mmio-interface: Mark as not user creatable

The mmio-interface device is not something we want to allow
users to create on the command line:
 * it is intended as an implementation detail of the memory
   subsystem, which gets created and deleted by that
   subsystem on demand; it makes no sense to create it
   by hand on the command line
 * it uses a pointer property 'host_ptr' which can't be
   set on the command line

Mark the device as not user_creatable to avoid confusion.

Signed-off-by: Peter Maydell <[email protected]>
Message-id: 1502807418[email protected]
Reviewed-by: Thomas Huth <[email protected]>