arm64: kvm: report original PAR_EL1 upon panic

If we call __kvm_hyp_panic while a guest context is active, we call
__restore_sysregs before acquiring the system register values for the
panic, in the process throwing away the PAR_EL1 value at the point of
the panic.

This patch modifies __kvm_hyp_panic to stash the PAR_EL1 value prior to
restoring host register values, enabling us to report the original
values at the point of the panic.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>