VIGS: Fix fence ack losses

VIGS: Fix fence ack losses

It's incorrect to have vblank enable/disable flag
in INT register, it can cause fence ack losses, consider
the following scenario:
1. Fence interrupt is set on host, fence_pending bit is
   set in INT register
2. vblank is turned off on guest, INT register is being
   written to and since fence_pending bit is 1 it's
   CLEARED on host
3. Now guest handles fence IRQ, but fence_pending bit
   is 0, thus, fence ack is lost

The solution is to have separate register - CON and
vblank enable/disable bit should be there

Change-Id: I1ff97d4cf8cddaec65dfe9d4c1afef3d901b4825