conf: split out virDomainFeaturesDefParse

The virDomainDefParseXML function has grown so large it broke the build:
../../src/conf/domain_conf.c:20362:1: error: stack frame size of 4168 bytes
in function 'virDomainDefParseXML' [-Werror,-Wframe-larger-than=]
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>