um: virt-pci: set device ready in probe()

Call virtio_device_ready() to make this driver work after commit
b4ec69d7e09 ("virtio: harden vring IRQ"), since the driver uses the
virtqueues in the probe function.  (The virtio core sets the device
ready when probe returns.)

Fixes: 8b4ec69d ("virtio: harden vring IRQ")
Fixes: 68f5d3f3

 ("um: add PCI over virtio emulation driver")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Message-Id: <20220610151203.3492541-1-vincent.whitchurch@axis.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Johannes Berg <johannes@sipsolutions.net>