softmmu/qdev-monitor: add error handling in qdev_set_id

qdev_set_id() is mostly used when the user adds a device (using
-device cli option or device_add qmp command). This commit adds
an error parameter to handle the case where the given id is
already taken.

Also document the function and add a return value in order to
be able to capture success/failure: the function now returns the
id in case of success, or NULL in case of failure.

The commit modifies the 2 calling places (qdev-monitor and
xen-legacy-backend) to add the error object parameter.

Note that the id is, right now, guaranteed to be unique because
all ids came from the "device" QemuOptsList where the id is used
as key. This addition is a preparation for a future commit which
will relax the uniqueness.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211008133442.141332-10-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>