spapr: Prevent boot CPU core removal

Boot CPU is assumed to be always present in QEMU code. So
until that assumptions are gone, deny removal request.
In another words, QEMU won't support boot CPU core hot-unplug.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
[dwg: Tweaked error message for clarity]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
