qapi: deprecate CpuInfoFast.arch

The TARGET_BASE_ARCH values from "configure" don't all map to the
@CpuInfoArch enum constants; in particular "s390x" from the former does
not match @s390 in the latter. Clients are known to rely on the @s390
constant specifically, so we can't change it silently. Instead, deprecate
the @CpuInfoFast.@arch member (in favor of @CpuInfoFast.@target) using the
regular deprecation process.

(No deprecation reminder is added to sysemu_target_to_cpuinfo_arch(): once
@CpuInfoFast.@arch is removed, the assignment expression that calls
sysemu_target_to_cpuinfo_arch() from qmp_query_cpus_fast() will have to
disappear; in turn the static function left without callers will also
break the build, thus it'll have to go.)

Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20180427192852.15013-6-lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>