qemu-option: avoid segfault if QemuOptsList == NULL

Signed-off-by: Peter Lieven <pl@kamp.de>
vl.c: fix segfault in iscsi options parsing

this patch fixes parsing of iscsi options such as initiator-name
passed to command line via -iscsi option group.

because iscsi options where registered too late qemu_find_opts
returned NULL leading to a segfault in qemu_opts_parse.

Signed-off-by: Peter Lieven <pl@kamp.de>