KVM: selftests: Implement perf_test_util more conventionally

It's not conventional C to put non-inline functions in header
files. Create a source file for the functions instead. Also
reduce the amount of globals and rename the functions to
something less generic.

Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20201218141734.54359-4-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>