qemu-option: Fix uninitialized value in append_option_parameter

When dest is NULL, i.e. a new copy of the list is created, we don't get a
properly terminated list after the realloc. Initialize it as an empty list.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
