Output the serial string of virtio disks

[liutgnu]

Previously, the serial string of virtio disks is not outputted. For example we can observe the issue if we have the following virtio disk configuration:

  <target dev="vda" bus="virtio"/>
  <serial>vblk1</serial>

Before:
       description: Virtual I/O device
       physical id: 0
       bus info: virtio@3
       logical name: /dev/vda
       size: 8GiB (8589MB)
       capabilities: partitioned partitioned:dos

After:
       description: Virtual I/O device
       physical id: 0
       bus info: virtio@3
       logical name: /dev/vda
       serial: vblk1
       size: 8GiB (8589MB)
       capabilities: partitioned partitioned:dos

Signed-off-by: Tao Liu [email protected]