virtio: Fix error handling in virtio_bus_device_plugged

For one thing we shouldn't continue if an error happened, for the other
two steps failing can cause an abort() in error_setg because we reuse
the same errp blindly.

Add error handling checks to fix both issues.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
