KVM: arm64: Clarify rationale for ZCR_EL1 value restored on guest exit

Expand comment clarifying why the host value representing SVE
vector length being restored for ZCR_EL1 on guest exit isn't the
same as it was on guest entry.

Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-21-tabba@google.com


Signed-off-by: Marc Zyngier <maz@kernel.org>