qemu-option: Don't reinvent append_option_parameters()

parse_option_parameters() may need to create a new option parameter list
from a template list.  Use append_option_parameters() instead of
duplicating the code.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>