How to ask a professor via email about possible postdoc positions? I have sent emails to a few (about 10) professors in different Computer Science research labs (related to my research interests) to see if they have any postdoc position available. 
However, I received no response at all. Of course, one assumption is that there wasn't any open position in those labs matching my CV. However, I was wondering if the structure of my emails is good/proper enough to motivate them to look into my CV?

Generally, I use the following structure with proper modifications according to the research theme of the person:


  Dear Prof. X
  I am XX, a Ph.D. candidate in the XX group of YY
  University, which is under the supervision of Prof. ZZ.
  
  I am about to finish my Ph.D. study (in the submission phase) which is
  about using different XX methods (X, Y, Z) for the problem of XX,  and
  currently, I am looking for postdoc opportunities relevant to my
  skills and interests.
  
  I studied your recent research and projects and I noticed you have a
  strong research theme in XX and YY.  I am really interested in your
  ongoing topics of ZZ and XYX, and I liked your publications which
  focus on DD and FF.
  
  Therefore, I am really interested to know if you have any open postdoc
  position at your lab regarding similar topics in XX and YY. I have
  attached my CV for your view, but I also welcome any opportunity to
  discuss the possibilities with you in any formal/informal meeting.
  
  Thank you very much for your time.


I write the third paragraph based on the research themes or topics in the research lab that I find interesting and relevant to my current skills.

I'm not sure if the length is good enough, and should I mention which specific topics from their ongoing/past research I am interested in? or I should let it be open to any possible discussion? Also, I have not met them before, so I introduce myself at the beginning.
Generally such positions are advertised and often must be. Adverts are placed in such places as professional journals/newsletters and in newsgroups related to the field in question. Given this practice, blind letters may not even be answered and you can assume that there are no positions available. 

Alternatively, if your relationship to your advisor is good enough s/he can ask around among acquaintances about the possible availability or future availability of such jobs. The advisor probably won't be ignored. 

A third possibility is to attend a suitable conference or other meeting and get to know people and explore the possibilities in person, making yourself known, but also trying to build a network of future collaborators. 

Your letter is probably fine, but it will probably be trashed. There are too many blind appeals for people to spend any time on them. In the old days of departmental secretaries it was different, but no more.