KVM_GET_ONE_REG and KVM_SET_ONE_REG return some error codes that
are not documented (but hopefully not surprising either).  To give
an indication of what these may mean, this patch adds brief
documentation.
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>