SpiceVmcPipeItem: use base PipeItem for ref counting
SpiceVmcPipeItem: use base PipeItem for ref counting

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

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