qemu: Fix command lines that are longer than 512 bytes

OSv image has 31.5 KB of space for command lines. Fix zero-padding for
command lines that are longer than 512 bytes.

Fixes #64.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #70 from penberg/qemu-cmdline-fix

qemu: Fix command lines that are longer than 512 bytes