intel/genxml/bits: Pull the function emit code into a helper block

intel/genxml/bits: Pull the function emit code into a helper block

The helper block is extremely general.  It takes an string property name
and an object that supports three methods: has_prop, iter_prop, and
get_prop.  This way we can easily generalize it to emit more different
types of getter functions.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>