Add pv_get_property and create generic internal _get_property function.

Add pv_get_property and create generic internal _get_property function.

We need to use a similar function for pv and lv properties, so just make
a generic _get_property() function that contains most of the required
functionality.  Also, add a check to ensure the field name matches the
object passed in by re-using report_type_t enum.  For pv properties,
the report_type might be either PVS or LABEL.

In addition, add 'const' to 'get' functions object parameter, but not
'set' functions.  Add _not_implemented_set() and _not_implemented_get()
functions.