xen/bootup: During bootup suppress XENBUS: Unable to read cpu state

When the initial domain starts, it prints (depending on the
amount of CPUs) a slew of
XENBUS: Unable to read cpu state
XENBUS: Unable to read cpu state
XENBUS: Unable to read cpu state
XENBUS: Unable to read cpu state

which provide no useful information - as the error is a valid
issue - but not on the initial domain. The reason is that the
XenStore is not accessible at that time (it is after all the
first guest) so the CPU hotplug watch cannot parse "availability/cpu"
attribute.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>