tools: make libxenhypfs interface more future proof

tools: make libxenhypfs interface more future proof

As compilers are free to choose the width of an enum they should be
avoided in stable interfaces when declaring a variable. So the
struct xenhypfs_dirent definition should be modified to have explicitly
sized members for type and encoding and the related enums should be
defined separately.

Additionally it is better to have a larger flags member in that struct
with the "writable" indicator occupying only a single bit. This will
make future additions easier.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>
Release-acked-by: Paul Durrant <paul@xen.org>