[FIX] account_payment: avoid using non-stored function field in domains.

Backport of following commits from trunk branch:
  8962 qdp-launchpad@openerp.com-20131030164925-c66d7jfx0x3546kt
  8974 qdp-launchpad@openerp.com-20131113100713-tvl30z5oqym22jqj