Move query-target and its return type TargetInfo from misc.json to
machine.json, where they are covered by MAINTAINERS section "Machine
core".  Also move its implementation from arch_init.c to
hw/core/machine-qmp-cmds, where it is likewise covered.

All users of SysEmuTarget are now in machine.json.  Move it there from
common.json.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190709152053.16670-3-armbru@redhat.com>