kvm_setup_guest_memory only does "madvise to QEMU_MADV_DONTFORK" and
is only called by ram_block_add, which actually is duplicate code.
Bonus: add simple comment for kvm_has_sync_mmu to make life easier.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Message-Id: <1473662096-32598-1-git-send-email-caoj.fnst@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>