qemu-iotests: Fix FilePaths docstring

When this class was extracted from FilePath, the docstring was not
updated for generating multiple files, and the example usage was
referencing unrelated file.

While fixing the docstring, add example for creating sockets, which
should use iotests.sock_dir instead of the default base_dir.

Fixes: de263986
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Message-Id: <20200828232152.205833-3-nsoffer@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>