lib: minor fixes to virDomainGetGuestInfo docs

lib: minor fixes to virDomainGetGuestInfo docs

Due to a typo, some of the field names didn't have closing quotes,
the information about the hostname was omitted and there was an
empty line missing after filesystem info description (which helps
our docs generator produce better looking HTML).

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>