KVM: selftests: Compile code with warnings enabled

So far the KVM selftests are compiled without any compiler warnings
enabled. That's quite bad, since we miss a lot of possible bugs this
way. Let's enable at least "-Wall" and some other useful warning flags
now, and fix at least the trivial problems in the code (like unused
variables).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>