KVM: Document errors for KVM_GET_ONE_REG and KVM_SET_ONE_REG

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: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/virtual/kvm/api.txt

@@ -1871,6 +1871,9 @@ Architectures: all
 Type: vcpu ioctl
 Parameters: struct kvm_one_reg (in)
 Returns: 0 on success, negative value on failure
+Errors:
+  ENOENT:   no such register
+  EINVAL:   other errors, such as bad size encoding for a known register
 
 struct kvm_one_reg {
        __u64 id;
@@ -2192,6 +2195,9 @@ Architectures: all
 Type: vcpu ioctl
 Parameters: struct kvm_one_reg (in and out)
 Returns: 0 on success, negative value on failure
+Errors:
+  ENOENT:   no such register
+  EINVAL:   other errors, such as bad size encoding for a known register
 
 This ioctl allows to receive the value of a single register implemented
 in a vcpu. The register to read is indicated by the "id" field of the