init.common: Fixing /dev/pts & /run creation

Previous commit was a little bit too agressive and broke the /dev/pts &
/run creation.

Was a little bit to early for such work :/
run_kvm: Fixing rights

Since the last few commits, the make test target was broken as some
writting rights were not present.

This commit make it work again.
make test: Adding FAKEIPMI

Since commit 2551b165206701abdad46c304071776fde42485f, the FAKEIPMI
argument is required to play with fake-ipmi devices.

This does make the test target working again.
try_match: Adding an inline documentation

When trying to debug a spec and an hw file it's always much boring
trying to remember what are the arguments orders while not reading the
code.

Instead of getting an exception, let's print a useful help message if
not the good amount of arguments are passed.