KVM: arm64: timer: Correctly handle EL1 timer emulation when !FEAT_ECV

KVM: arm64: timer: Correctly handle EL1 timer emulation when !FEAT_ECV

Both Wei-Lin Chang and Volodymyr Babchuk report that the way we
handle the emulation of EL1 timers with NV is completely wrong,
specially in the case of HCR_EL2.E2H==0.

There are three problems in about as many lines of code:

- With E2H==0, the EL1 timers are overwritten with the EL1 state,
  while they should actually contain the EL2 state (as per the timer
  map)

- With E2H==1, we run the full EL1 timer emulation even when ECV
  is present, hiding a bug in timer_emulate() (see previous patch)

- The comments are actively misleading, and say all the wrong things.

This is only attributable to the code having been initially written
for FEAT_NV, hacked up to handle FEAT_NV2 *in parallel*, and vaguely
hacked again to be FEAT_NV2 only. Oh, and yours truly being a gold
plated idiot.

The fix is obvious: just delete most of the E2H==0 code, have a unified
handling of the timers (because they really are E2H agnostic), and
make sure we don't execute any of that when FEAT_ECV is present.

Fixes: 4bad3068cfa9f ("KVM: arm64: nv: Sync nested timer state with FEAT_NV2")
Reported-by: Wei-Lin Chang <r09922117@csie.ntu.edu.tw>
Reported-by: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Link: https://lore.kernel.org/r/fqiqfjzwpgbzdtouu2pwqlu7llhnf5lmy4hzv5vo6ph4v3vyls@jdcfy3fjjc5k
Link: https://lore.kernel.org/r/87frl51tse.fsf@epam.com
Tested-by: Dmytro Terletskyi <dmytro_terletskyi@epam.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250204110050.150560-3-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>