When read, this returns the number of PVST spanning trees
active in the switch, which should correspond to the number
of entries in ctPvstStpTable. When written with 4094, it
should create a spanning tree for each currently configured
VLAN. If a switch cannot create as many spanning trees as
configured VLANs, this write should return an error.
Writing any value other than 4094 should return an
error.