kvm tools: Scale guest RAM size by CPU count

kvm tools: Scale guest RAM size by CPU count

This patch increases default RAM size to 256 for one CPU and introduces RAM
size linear scaling based on CPUs as suggested by Ingo Molnar:

     64MB*(nr_cpus + 3)

     ------------------
       1 CPUs:   256 MB
       2 CPUs:   320 MB
       3 CPUs:   384 MB
       4 CPUs:   448 MB
       5 CPUs:   512 MB
       6 CPUs:   576 MB
       7 CPUs:   640 MB
       8 CPUs:   704 MB
       9 CPUs:   768 MB
      10 CPUs:   832 MB
      11 CPUs:   896 MB
      12 CPUs:   960 MB
      13 CPUs:  1024 MB
      14 CPUs:  1088 MB
      15 CPUs:  1152 MB
      16 CPUs:  1216 MB

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@kernel.org>