hw/core: rebase sysbus_get_fw_dev_path() to g_strdup_printf()

This is done mainly for improving readability, and in preparation for the
next patch, but Markus pointed out another bonus for the string being
returned:

"No arbitrary length limit. Before the patch, it's 39 characters, and the
code breaks catastrophically when qdev_fw_name() is longer: the second
snprintf() is called with its first argument pointing beyond path[], and
its second argument underflowing to a huge size."

Cc: qemu-stable@nongnu.org
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
