KVM: arm64: Fix hypercall bitmap writeback when vcpus have already run

We generally want to disallow hypercall bitmaps being changed
once vcpus have already run. But we must allow the write if
the written value is unchanged so that userspace can rewrite
the register file on reboot, for example.

Without this, a QEMU-based VM will fail to reboot correctly.

The original code was correct, and it is me that introduced
the regression.

Fixes: 05714cab ("KVM: arm64: Setup a framework for hypercall bitmap firmware registers")
Signed-off-by: Marc Zyngier <maz@kernel.org>