qdev: Don't abort() in case globals can't be set

qdev: Don't abort() in case globals can't be set

It would be much better if we didn't terminate QEMU inside
device_post_init(), but at least exiting cleanly is better than aborting
and dumping core.

Before this patch:

    $ qemu-system-x86_64 -global cpu.xxx=y
    qemu-system-x86_64: Property '.xxx' not found
    Aborted (core dumped)

After this patch:

    $ qemu-system-x86_64 -global cpu.xxx=y
    qemu-system-x86_64: Property '.xxx' not found

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-By: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>