In my "build everything" tree, changing sysemu/sysemu.h triggers a
recompile of some 5400 out of 6600 objects (not counting tests and
objects that don't depend on qemu/osdep.h).

hw/qdev-core.h includes sysemu/sysemu.h since recent commit e965ffa7
"qdev: add qdev_add_vm_change_state_handler()".  This is a bad idea:
hw/qdev-core.h is widely included.

Move the declaration of qdev_add_vm_change_state_handler() to
sysemu/sysemu.h, and drop the problematic include from hw/qdev-core.h.

Touching sysemu/sysemu.h now recompiles some 1800 objects.
qemu/uuid.h also drops from 5400 to 1800.  A few more headers show
smaller improvement: qemu/notify.h drops from 5600 to 5200,
qemu/timer.h from 5600 to 4500, and qapi/qapi-types-run-state.h from
5500 to 5000.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NAlistair Francis <alistair.francis@wdc.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190812052359.30071-28-armbru@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>