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

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 <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stewart Smith <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>