INT 2F - MS WINDOWS - SWITCH VMs AND CALLBACK
        AX = 1685h
        BX = VM ID of virtual machine to switch to
        CX = flags
                bit 0 wait until interrupts enabled
                bit 1 wait until critical section unowned
                bits 2-15 reserved (zero)
        DX:SI = priority boost (see VMM.INC)
        ES:DI -> FAR procedure to callback
Return: CF set on error
            AX = error code
                01h invalid VM ID
                02h invalid priority boost
                03h invalid flags
        CF clear if successful
            event will be or has been called
Notes:  some DOS devices, such as networks, need to call functions in a
          specific VM. This call forces the appropriate VM to be installed.
        the callback procedure must preserve all registers and return with IRET
SeeAlso: INT 15/AX=1117h