test-runner: Use '-cpu host' in kvm mode

When KVM is available, use the -cpu host option.  This is useful on
distributions that might use architecture specific optimizations, which
might not be available on the default machine being emulated by qemu.

For example, a Gentoo box compiled with "-march=core-avx2" would result
in several binaries not executing properly inside the qemu environment
due to instruction set mismatches.

tools/test-runner.c

@@ -301,8 +301,11 @@ static void start_qemu(void)
 	argv[pos++] = "-append";
 	argv[pos++] = (char *) cmdline;
 
-	if (has_virt)
+	if (has_virt) {
+		argv[pos++] = "-cpu";
+		argv[pos++] = "host";
 		argv[pos++] = "-enable-kvm";
+	}
 
 	argv[pos] = NULL;