Now that we have the ability to specify arbitrary qemu
command-line parameters in the XML, use it to handle unknown
command-line parameters when doing a native-to-xml conversion.

Changes since v1:
 - Rename num_extra to num_args
 - Fix up a memory leak on an error path

Changes since v2:
 - Add a VIR_WARN when adding the argument via qemu:arg

Changes since v3:
 - None
Signed-off-by: NChris Lalancette <clalance@redhat.com>