+++ This bug was initially created as a clone of Bug #1151903 +++

Description of problem:
Some virt tools have a --csv option, when it works with other options the output maybe meaningless. Take virt-diff for example. When i run virt-diff with --csv option, i found the output is not a csv file, the output contains many lines which have the same content, and the field number in each line is not equal. It is hard to read such output and import it to a database. If virt-diff provide --csv option then it should make it easy to use, if it combine with other options then it should make sure the output is meaningful. The --csv problem may also exist in other virt tools or combine with other options, such as:
#virt-ls -lR -a rhel.img / --uids --csv |awk -F, '{print NF}' |sort
#virt-diff -a rhel6.6.img -A rhel.img --times --csv  |awk -F, '{print NF}'|sort


Version-Release number of selected component (if applicable):
libguestfs-1.27.61-1.1.el7


How reproducible:
100%


Steps to Reproduce:
1. Get a rhel guest image rhel6.6.img
2. #cp rhel6.6.img rhel.img
3. Boot the guest with rhel.img and shutdown it
4. #virt-diff -a rhel6.6.img -A rhel.img --atime --csv
5. #virt-diff -a rhel6.6.img -A rhel.img --atime --csv  |awk -F, '{print NF}' |sort


Actual results:
.....
#,changed:,st_ino,st_atime_sec,st_mtime_sec,st_ctime_sec
-,s,0666,0,/var/spool/postfix/public/cleanup
+,s,0666,0,/var/spool/postfix/public/cleanup
#,changed:,st_ino,st_atime_sec,st_mtime_sec,st_ctime_sec
-,s,0666,0,/var/spool/postfix/public/flush
+,s,0666,0,/var/spool/postfix/public/flush
#,changed:,st_ino,st_atime_sec,st_mtime_sec,st_ctime_sec
-,p,0622,0,/var/spool/postfix/public/pickup
+,p,0622,0,/var/spool/postfix/public/pickup
#,changed:,st_ino,st_atime_sec,st_mtime_sec,st_ctime_sec
-,p,0622,0,/var/spool/postfix/public/qmgr
+,p,0622,0,/var/spool/postfix/public/qmgr
#,changed:,st_ino,st_atime_sec,st_mtime_sec,st_ctime_sec
-,s,0666,0,/var/spool/postfix/public/showq
+,s,0666,0,/var/spool/postfix/public/showq
#,changed:,st_ino,st_atime_sec,st_mtime_sec,st_ctime_sec

#virt-diff -a rhel6.6.img -A rhel.img --atime --csv  |awk -F, '{print NF}' |sort
....
5
5
3
5
5
3
5
5
3
6
6
3
5
5
....


Expected results:
The best way for the csv output is only contains one header in the first line instead of print it many times and output the field even if the field have no value, i.e. each line should have the same number of field.


Additional info: