Modern distributions place xattr.h in /usr/include/sys, and fold
libattr.so into libc.  They also don't have an ENOATTR.

Make configure detect this, and add a qemu-xattr.h file that
directs the #include to the right place.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>