Get and Enable Virtual Interrupt State

Enables the virtual interrupt flag and returns the previous
state of the virtual interrupt flag.

Call With:

   AX     = 0901H

Returns:

   Virtual interrupts enabled
   Carry flag= clear (this function always succeeds)
   AL     = 0 if virtual interrupts were previously disabled
          = 1 if virtual interrupts were previously enabled

Notes:

o  AH is not changed by this function.  Therefore, the
   previous state can be restored by simply executing
   another Int 31H.  See Int 31H Fn 0900H.

o  A client that does not need to know the prior interrupt
   state can execute the STI instruction rather than calling
   this function.  The instruction may be trapped by the
   host and should be assumed to  be very slow.