df: Move variable decl to top of function.

Since this code is no longer conditional, we don't need to have the
extra { ... } inside the case statement.

This tidies up commit 2f0a4d7c18e622c48e33edc3f80dbd593c6398bc.
fish: options: Format server name as "tcp:host[:port]".

This is safer, because otherwise a URI could contain some clever
"unix:..." string as the hostname, tricking qemu into opening a Unix
domain socket at an uncontrolled location.

This fixes commit 349300af08be9ae9f3d7259e688e2ee5318f2171.
fish: options: Allow Unix domain sockets to be specified in --add URIs.

You can now use a URI such as:

 guestfish -a nbd://?socket=/tmp/sock

(a NBD connection to Unix domain socket /tmp/sock).