Add multikey API:

 * virTypedParamsFilter that filters all the parameters with specified name.
 * virTypedParamsGetStringList that returns a list with all the values for
   specified name and string type.
Signed-off-by: NPavel Boldin <pboldin@mirantis.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>