phyp: use consistent return string handling

phyp: use consistent return string handling

Use the name 'ret' for all phypExec results, to make it easier
to wrap phypExec.  Don't allow a possibly NULL ret through printf.

* src/phyp/phyp_driver.c (phypBuildVolume, phypDestroyStoragePool)
(phypBuildStoragePool, phypBuildLpar): Avoid NULL dereference.
(phypInterfaceDestroy): Avoid redundant free.
(phypVolumeLookupByPath, phypVolumeGetPath): Use consistent
naming.