pressure-vessel: Override log handler for pressure-vessel log domain

When this code moved into the shared library, it wasn't adapted to take into account pressure-vessel and steam-runtime-tools being separate log domains. The result was that PRESSURE_VESSEL_VERBOSE actually only made libsteam-runtime-tools verbose, and info/debug messages from the pressure-vessel directory were lost.

Fixes: 2194b59a "Move pressure-vessel's logging setup into library code"