grub-shell: Pass -no-pad to xorriso when building floppy images
The floppy images built by grub-shell are currently just over the
floppy limit, and the Debian patch set plus the 915resolution extra
bring this up to the point where GRUB tries to actually read past
the floppy limit, breaking fddboot_test. Passing -no-pad buys us
300KiB, which should keep us going for a while.
* tests/util/grub-shell.in: Pass -no-pad to xorriso when building
floppy images, saving 300KiB.