multiboot: Quote filename in error message

Quote filename in error message to spot possible whitespace character in
the filename and make error message more meaningful.

Signed-off-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>