gdb: add 'osv info virtio'

Dumps virtio drivers and the state of their queues.

Sample output:

(gdb) osv info virtio
virtio::virtio_net at 0xffffc0003ff0ec00
  queue 0 at 0xffffc0003ff2ba00
    avail g=0xbe09 h=0xbe09 (0)
    used   h=0xbe09 g=0xbd11 (248)
    used notifications: enabled
  queue 1 at 0xffffc0003ff2bc00
    avail g=0xc951 h=0xc951 (0)
    used   h=0xc951 g=0xc8fd (84)
    used notifications: enabled
  queue 2 at 0xffffc0003ff2bd00
    avail g=0x0 h=0x0 (0)
    used   h=0x0 g=0x0 (0)
    used notifications: enabled
virtio::virtio_blk at 0xffffc0003fefd400
  queue 0 at 0xffffc0003fee5100
    avail g=0x15f h=0x15f (0)
    used   h=0x15f g=0x15f (0)
    used notifications: enabled

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>