This patch optimizes the guest page table walk by using get_user()
instead of copy_from_user().

With this patch applied, paging64_walk_addr_generic() has become
about 0.5us to 1.0us faster on my Phenom II machine with NPT on.
Signed-off-by: NTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: NAvi Kivity <avi@redhat.com>