qapi: fill in CpuInfoFast.arch in query-cpus-fast

qapi: fill in CpuInfoFast.arch in query-cpus-fast

* Commit ca230ff33f89 added the @arch field to @CpuInfoFast, but it failed
  to set the new field in qmp_query_cpus_fast(), when TARGET_S390X was not
  defined. The updated @query-cpus-fast example in "qapi-schema.json"
  showed "arch":"x86" only because qmp_query_cpus_fast() calls g_malloc0()
  to allocate @CpuInfoFast, and the CPU_INFO_ARCH_X86 enum constant is
  generated with value 0.

  All @arch values other than @s390 implied the @CpuInfoOther sub-struct
  for @CpuInfoFast -- at the time of writing the patch --, thus no fields
  other than @arch needed to be set when TARGET_S390X was not defined. Set
  @arch now, by copying the corresponding assignments from
  qmp_query_cpus().

* Commit 25fa194b7b11 added the @riscv enum constant to @CpuInfoArch (used
  in both @CpuInfo and @CpuInfoFast -- the return types of the @query-cpus
  and @query-cpus-fast commands, respectively), and assigned, in both
  return structures, the @CpuInfoRISCV sub-structure to the new enum
  value.

  However, qmp_query_cpus_fast() would not populate either the @arch field
  or the @CpuInfoRISCV sub-structure, when TARGET_RISCV was defined; only
  qmp_query_cpus() would.

  Assign @CpuInfoOther to the @riscv enum constant in @CpuInfoFast, and
  populate only the @arch field in qmp_query_cpus_fast(). Getting CPU
  state without interrupting KVM is an exceptional thing that only S390X
  does currently. Quoting Cornelia Huck <cohuck@redhat.com>, "s390x is
  exceptional in that it has state in QEMU that is actually interesting
  for upper layers and can be retrieved without performance penalty". See
  also
  <https://www.redhat.com/archives/libvir-list/2018-February/msg00121.html>.

Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Viktor VM Mihajlovski <mihajlov@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Fixes: ca230ff33f89bf7102cbfbc2328716da6750aaed
Fixes: 25fa194b7b11901561532e435beb83d046899f7a
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180427192852.15013-2-lersek@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>