target-ppc: Force CPU threads count to be a power of 2

PowerPC kernel expects the number of SMT threads in a core to be a power
of 2. Since QEMU doesn't enforce this, it leads to an early guest kernel
crash if invalid threads count is specified.

Prevent this crash and make it a graceful exit from QEMU itself by
validating the user-supplied threads count.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>