[WIP] hr_holidays: Leaves for partial time are wrongly counted

Steps to reproduce the bug:

- Let's consider an employee E that doesn't work on Monday morning
- Create a half day leave request LR for E on a Monday afternoon
- Save

Bug:

The duration of LR was counted as one day instead of 0.5

opw:2491784

closes odoo/odoo#68977

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>