qga/schema: fix documentation for GuestOSInfo

The documentation for kernel-version and kernel-release on Windows was
swapped.

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>