docs: split typedef and struct definition for apibuild.py

docs: split typedef and struct definition for apibuild.py

The members of struct virSecurityLabel and struct virSecurityModel
were not shown in the libvirt API docs because the corresponding
<field> elements were missing from the libvirt-api.xml.

The reason is that apibuild.py does not cope well with typedef's
using inline struct definitions. It fails to associate the comment
with the typedef and because of this refuses to write out the
field of the struct.