To ease converting the net->model value to an enum, add
the wrapper functions:

virDomainNetGetModelString
virDomainNetSetModelString
virDomainNetStreqModelString
virDomainNetStrcaseeqModelString
Acked-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>