target-i386: Fix X86CPU error handling

Error **errp argument is not for emitting warnings, it means an error
has occurred and the caller should not make any assumptions about the
state of other return values (unless otherwise documented).

Therefore cpu_x86_create() must unref the new X86CPU itself, and
pc_new_cpu() must check for an Error rather than NULL return value.

While at it, clean up a superfluous NULL check.

Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: qemu-stable@nongnu.org
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
