Determines number of USB devices known to the Parallels Service.
PRL_RESULT PrlDispCfg_GetUsbIdentityCount( PRL_HANDLE hDispCfg, PRL_UINT32_PTR pnUsbIdentityCount );
PrlApiDisp.h
PRL_RESULT. Possible values are:
PRL_ERR_INVALID_ARG - either hDispCfg is not a valid PHT_DISP_CONFIG
PRL_SUCCESS - function completed successfully