virtfs-proxy-helper: Fix compilation on newer systems

Include file attr/xattr.h is not available on "newer" systems
(for example Fedora 12 or Debian Squeeze).

See comments in qemu-xattr.h for more information.
This file handles the system dependencies automatically.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>