qemu-file: Fix qemu_put_compression_data flaw

Current qemu_put_compression_data can only work with no writable
QEMUFile, and can't work with the writable QEMUFile. But it does
not provide any measure to prevent users from using it with a
writable QEMUFile.

We should fix this flaw to make it works with writable QEMUFile.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Suggested-by: Juan Quintela <quintela@redhat.com>
Message-Id: <1462433579-13691-5-git-send-email-liang.z.li@intel.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
