qapi: add 'query-target' command to return target arch

qapi: add 'query-target' command to return target arch

Add a 'query-target' QAPI command to allow management applications
to determine what target architecture a QEMU binary is emulating
without having to parse the binary name or -help output

  $ qmp-shell -p /tmp/qemu
  (QEMU) query-target
  {   u'return': {   u'arch': u'x86_64' }}

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>