conf: drop redundant parameter to chain lookup
    
    The original chain lookup code had to pass in the starting name,
    because it was not available in the chain.  But now that we have
    added fields to the struct, this parameter is redundant.
    
    * src/util/virstoragefile.h (virStorageFileChainLookup): Alter
    signature.
    * src/util/virstoragefile.c (virStorageFileChainLookup): Adjust
    handling of top of chain.
    * src/qemu/qemu_driver.c (qemuDomainBlockCommit): Adjust caller.
    * tests/virstoragetest.c (testStorageLookup, mymain): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>