x86: Correct copying of BIOS mode information

x86: Correct copying of BIOS mode information

This is copying beyond the end of the destination buffer. Correct the code
by using the size of the  vesa_mode_info struct. We don't need to copy the
rest of the bytes in the buffer.

This long-standing bug prevents virtio bootdevs working correctly on
qemu-x86 at present.

Fixes: 0ca2426beae ("x86: Add support for running option ROMs natively")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com> # qemu-x86_64