conf: Split out domain features formatting from virDomainDefFormatInternal

Pure code motion of code for formatting domain features to a function
called virDomainDefFormatFeatures. Best viewed with the '--patience'
option for git show.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>