builder: Add test *.xz files to CLEANFILES.
tests/nbd: Avoid warning when tests are skipped.

Since END{} clause is always called,  might not be defined if
we exit early.  Move the whole clause to the top.
builder: Add -v option to test so we get all debugging output.
builder: Fix --format/--output-format arguments to virt-resize subcommand.

The 'virt-builder --format' option translates to the
'virt-resize --output-format' option, because it specifies the output
format.

On the contrary, the 'virt-resize --format' option (the input format)
is always 'raw'.
uml: Fix virt-builder test.

Note comment in the test about networks.