Control relay from 2 circuits
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A latching mains power-on circuit.

The circuit of Figure 1 might do the trick. 


A push to make switch energises the PSU.
A contact of the relay latches the power on.
A push to break switch unlatches the circuit to power down. Note that this may have to be held for several seconds if the power supply is slow to shut down - particularly when lightly loaded.