smartcard: Use base PipeItem for MsgItem refcounting
smartcard: Use base PipeItem for MsgItem refcounting

This allows to reuse pipe_item_{ref, unref} rather than
reimplementing them in smartcard.c

SPICE/spice,fgouget/spice,SPICE/spice,SPICE/spice,fgouget/spice,fgouget/spice,fgouget/spice