KVM: arm64: Introduce accessor for ctxt->sys_reg

In order to allow the disintegration of the per-vcpu sysreg array,
let's introduce a new helper (ctxt_sys_reg()) that returns the
in-memory copy of a system register, picked from a given context.

__vcpu_sys_reg() is rewritten to use this helper.

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