add "/usr/bin/kvm" to search options

debian systems have a /usr/bin/kvm script for backward compatibility,
which calls the real qemu-<name> binary.

It won't hurt if we add it to this "list of possible kvm executable names".
Merge pull request #2 from mfournier/debian-add-path-to-kvm

add "/usr/bin/kvm" to search options