Generate VCD for each trace of cover property

EBMC can generate traces for each cover property using `--trace` switch, and each individual trace gets printed. Only 1 VCD file is getting generated with the `--vcd` switch, and I am not able to view each trace in a waveform viewer. 

It would be great if each trace gets generated as a separate VCD file, like how SBY in Yosys project currently does (https://github.com/YosysHQ/sby).