runqemu-gen-tapdevs: Refactoring

The changes are mostly about early exit which causes indentation changes;
check with `git diff -w`. Another change is the check for ip by simply
calling it and deciding upon the exit code, if it's fine or not.

Signed-off-by: Jörg Sommer <joerg.sommer@navimatix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>