qapi: discriminate CpuInfoFast on SysEmuTarget, not CpuInfoArch

qapi: discriminate CpuInfoFast on SysEmuTarget, not CpuInfoArch

Add a new field @target (of type @SysEmuTarget) to the output of the
@query-cpus-fast command, which provides more information about the
emulation target than the field @arch (of type @CpuInfoArch). Make @target
the new discriminator for the @CpuInfoFast return structure. Keep @arch
for compatibility.

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>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180427192852.15013-5-lersek@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>