qga: escape cmdline args when registering win32 service (CVE-2013-2231)

Reported-by: Lev Veyde <lveyde@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
