vgsp: X/GL frame-pacer mirror (sema_xw_mirror) - unblocks the 10s/frame stall

r861 evidence: the X server's present path spins in a 10s-timeout wait
((EE) NVIDIA(GPU-0): WAIT (2, 8, 0x8000, <stuck 0xd4..>, <driver counter
+0x70/frame>)) comparing (progress_word & 0x3ffff) << 2 against its own
submitted-byte counter; the progress word (lone u32 0x140235 in an
otherwise-zero guest page) never advances because the emulation does not
execute the 2D method streams -> glxgears 0.09 FPS. A gdb write of the
word to 0x140000 | (counter >> 2) made the wait complete in 3 ms
(WAIT (0, ... <equal>, <equal>) immediately after the timeout print) and
silenced all further timeouts. This adds a 3 ms kthread that mirrors the
driver's counter word into the progress word; both guest PAs are injected
per boot via the sema_xw_mirror parameter (0,0 stops). Demo-path tuning
aid; no product logic depends on it.

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