KVM: arm64: Don't hijack guest context MDSCR_EL1

Stealing MDSCR_EL1 in the guest's kvm_cpu_context for external debugging
is rather gross. Just add a field for this instead and let the context
switch code pick the correct one based on the debug owner.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20241219224116.3941496-15-oliver.upton@linux.dev


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