KVM: x86: Add comment on vcpu_enter_guest()'s return value

Giving proper names to the 0 and 1 was once suggested.  But since 0 is
returned to the userspace, giving it another name can introduce extra
confusion.  This patch just explains the meanings instead.

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>