sysemu: support up to 1024 vCPUs

Some systems can already provide more than 255 hardware threads.

Bumping the QEMU limit to 1024 seems reasonable:
- it has no visible overhead in top;
- the limit itself has no effect on hot paths.

Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
