INT 15 - DESQview v2.26+ - "ASSERTVIR" - CONTROL 386 SCREEN VIRTUALIZATION
        AX = DE21h
        BX = new state
            0000h turn off
            nonzero turn on
Return: BX = old state of virtualization
Notes:  this API call may be made from within a hardware interrupt handler
        under DV 2.40 and 2.42, this call appears to have no effect and always
          returns a nonzero value in BX which appears to be the offset within
          the DV common memory segment of the caller's task object; it may
          only have an effect within a hardware interrupt handler
SeeAlso: AX=DE17h