Update bhyveBuildDiskArgStr to support volumes:

 - Make virBhyveProcessBuildBhyveCmd and
   virBhyveProcessBuildLoadCmd take virConnectPtr as the
   first argument instead of bhyveConnPtr as virConnectPtr is
   needed for virStorageTranslateDiskSourcePool,
 - Add virStorageTranslateDiskSourcePool call to
   virBhyveProcessBuildBhyveCmd and
   virBhyveProcessBuildLoadCmd,
 - Allow disks of type VIR_STORAGE_TYPE_VOLUME