Change the devs array into a linked list, and add a scsi_device_find
function to navigate the children list instead.  This lets the SCSI
bus use more complex addressing, and HBAs can talk to the correct device
when there are multiple LUNs per target.

scsi_device_find may return another LUN on the same target if none is
found that matches exactly.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>