KVM: arm/arm64: Fix vGIC documentation

Correct some old mistakes in the API documentation:

1. VCPU is identified by index (using kvm_get_vcpu() function), but
   "cpu id" can be mistaken for affinity ID.
2. Some error codes are wrong.

  [ Slightly tweaked some grammer and did some s/CPU index/vcpu_index/
    in the descriptions.  -Christoffer ]

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>