Stop relying on specific values of PED_PARTITION_* enum

The expressions used in the call to Set() were comparing
lp_partition->type to 0 for the type parameter and passing it as a bool
for the inside_extended parameter.  Libparted lp_partition->type is
actually an enumeration.  The code was only working because of the
specific values assigned to the symbolic names, PED_PARTITION_NORMAL = 0
and PED_PARTITION_EXTENDED is non-zero (true).

Make the code use the symbolic names and not depend on the actual
enumeration values, which should be considered changeable and private to
libparted.