hw/acpi: Fix legacy CPU plug error API violations

hw/acpi: Fix legacy CPU plug error API violations

legacy_acpi_cpu_plug_cb() dereferences @errp when
acpi_set_cpu_present_bit() fails.  That's wrong; see the big comment
in error.h.  Introduced in commit cc43364de7 "acpi/cpu-hotplug:
introduce helper function to keep bit setting in one place".

No caller actually passes null, and acpi_set_cpu_present_bit() can't
actually fail.

Fix anyway: drop acpi_set_cpu_present_bit()'s @errp parameter.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20191204093625.14836-7-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>