KVM: arm64: Allow EL1 control registers to be accessed from the CPU state

KVM: arm64: Allow EL1 control registers to be accessed from the CPU state

As we are about to plug the SW PTW into the EL1-only code, we can
no longer assume that the EL1 state is not resident on the CPU,
as we don't necessarily get there from EL2 traps.

Turn the __vcpu_sys_reg() access on the EL1 state into calls to
the vcpu_read_sys_reg() helper, which is guaranteed to do the
right thing.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>