build: use pythondir for vdsmapi

Change-Id: I8077efea03adb7d018c35cfd81b121b9e47d3e98
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17249
Reviewed-by: Dan Kenigsberg <danken@redhat.com>