target-ppc: kvm: Fix memory overflow issue about strncat()

strncat() will append additional '\0' to destination buffer, so need
additional 1 byte for it, or may cause memory overflow, just like other
area within QEMU have done.

And can use g_strdup_printf() instead of strncat(), which may be more
easier understanding.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
