fw_cfg: Fix -boot bootsplash error checking

fw_cfg: Fix -boot bootsplash error checking

fw_cfg_bootsplash() gets option parameter "splash-time"
with qemu_opt_get(), then converts it to an integer by hand.
It neglects to check that conversion for errors. This is
needlessly complicated and error-prone. But as "splash-time
not specified" is not the same as "splash-time=T" for any T,
we need use qemu_opt_get() to check if splash time exists.
This patch also make the qemu exit when finding or loading
splash file failed.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1542777026-2788-2-git-send-email-liq3ea@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>