Bugfix for loading multiboot kernels

Bugfix for loading multiboot kernels

This patch fixes a bug in rom_copy introduced by
commit d60fa42e8bae39440f997ebfe8fe328269a57d16.

rom_copy failed to load roms with a "datasize" of 0.
As a result, multiboot kernels were not loaded correctly
when they contain a segment with a "file size" of 0.

https://bugs.launchpad.net/qemu/+bug/1208944

Signed-off-by: Martijn van den Broek <martijn.vdbrk@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: CAG1x_oET1u3TMPu3r_zzd3ZXsTWQLiaM0zAc+RkHFCwvJjGOvg@mail.gmail.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>