migration: refine ram_save_compressed_page

Use qemu_put_compression_data to do the compression directly
instead of using do_compress_ram_page, avoid some data copy.
very small improvement, at the same time, add code to check
if the compression is successful.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Message-Id: <1462433579-13691-6-git-send-email-liang.z.li@intel.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
