[FIX] Report: get_pdf:  don't miss main elements when parsing html

Main elements can have more than one class, thus can be ignored by the previous
naive xpath. This patch brings a stronger xpath.
[FIX] product: pricelists based on the supplier prices now return first supplier price if partner not in suppliers instead of returning 0

Without this, sale pricelists cannot be based on supplier prices as the partner is never a supplier so the price would be 0
Note: behavior change ok with fp
Merge branch '8.0' into 8.0-pos-backports-fva