KVM: arm64: Propagate vcpu into read_id_reg()

Architecture features that are conditionally visible to the guest
will require run-time checks in the ID register accessor functions.
In particular, read_id_reg() will need to perform checks in order
to generate the correct emulated value for certain ID register
fields such as ID_AA64PFR0_EL1.SVE for example.

This patch propagates vcpu into read_id_reg() so that future
patches can add run-time checks on the guest configuration here.

For now, there is no functional change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>