x86: small fixes to pcpu platform op handling

x86: small fixes to pcpu platform op handling

XENPF_get_cpuinfo should init the flags output field rather than only
modify it.

XENPF_cpu_online must check for the input CPU number to be in range.

XENPF_cpu_offline must also do that, and should also reject attempts to
offline CPU 0 (this fails in cpu_down() too, but preventing this here
appears more correct given that the code here calls
continue_hypercall_on_cpu(0, ...), which would be flawed if cpu_down()
would ever allow bringing down CPU 0 (and a distinct error code is
easier to deal with when debugging issues).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>