codegen: ptypes.py: keep attribute names in sets
codegen: ptypes.py: keep attribute names in sets

This patch changes the type of 'valid_attributes' and
'attributes_with_arguments'.
Both of them are list of different strings and are kept in sets.

This was the intention of the original code, but this patch
use a specific set([ strings ]) format, instead of { strings }.

This fixes the build for me on RHEL-6 (python-2.6.6).
Build error is:

File "/home/ulublin/git/spice/spice-common/python_modules/ptypes.py",
line 67
      'end',
           ^
SyntaxError: invalid syntax

fgouget/spice-common,SPICE/spice-common,fgouget/spice-common,fgouget/spice-common,SPICE/spice-common,fgouget/spice-common,SPICE/spice-common