libxl: add is_default checkers for string and timer_mode types

libxl: add is_default checkers for string and timer_mode types

Those types are missing a helper to check whether a definition of the
type holds the default value. This will be required by a later patch
that will implement deprecation of fields inside of a libxl type.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>