storage: better tests of lookup

storage: better tests of lookup

Add some more tests of what happens when we restrict a lookup
to begin at a point in the middle of a chain.  In particular,
we want to ensure that a parent is not found when starting at
the child.  This commit also demonstrates that we have a slight
difference in behavior on what parent we report when filtering
is in effect; as the determination of the parent affects the
code in block commit, exposing this in the testsuite will help
justify changes in future patches that tweak the semantics of
what lookups are allowed.

* tests/virstoragetest.c (testStorageLookup): Test user input.
(TEST_LOOKUP_TARGET): Add parameter.
(mymain): Add lookup tests.

Signed-off-by: Eric Blake <eblake@redhat.com>