hw/9pfs: Improve portability to older systems

hw/9pfs: Improve portability to older systems

handle fs driver require a set of newly added syscalls. Don't
Compile handle FS driver if those syscalls are not available.
Instead of adding #ifdef for all those syscalls we check for
open by handle syscall. If that is available then rest of the
syscalls used by the driver should be available.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>