xen: Fix xen-legacy-backend qdev types

xen: Fix xen-legacy-backend qdev types

xen-sysdev is a TYPE_SYS_BUS_DEVICE.  bus_type should not be changed so
that it can plug into the System bus.  Otherwise this assert triggers:
qemu-system-i386: hw/core/qdev.c:102: qdev_set_parent_bus: Assertion
`dc->bus_type && object_dynamic_cast(OBJECT(bus), dc->bus_type)'
failed.

TYPE_XENBACKEND attaches to TYPE_XENSYSBUS, so its class_init needs to
be set accordingly to attach the qdev.  Otherwise the following assert
triggers:
qemu-system-i386: hw/core/qdev.c:102: qdev_set_parent_bus: Assertion
`dc->bus_type && object_dynamic_cast(OBJECT(bus), dc->bus_type)'
failed.

TYPE_XENBACKEND is not a subclass of XEN_XENSYSDEV, so it's parent
is just TYPE_DEVICE.  Change that.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Paul Durrant <pdurrant@amazon.com>
Fixes: 81cb05732efb ("qdev: Assert devices are plugged into a bus that can take them")
Message-Id: <20200624121939.10282-1-jandryuk@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>