qdev: bus walker + qdev_device_add()

qdev: bus walker + qdev_device_add()

This patch implements a parser and qdev tree walker for bus paths and
adds qdev_device_add on top of this.

A bus path can be:
  (1) full path, i.e. /i440FX-pcihost/pci.0/lsi/scsi.0
  (2) bus name, i.e. "scsi.0".  Best used together with id= to make
      sure this is unique.
  (3) relative path starting with a bus name, i.e. "pci.0/lsi/scsi.0"

For the (common) case of a single child bus being attached to a device
it is enougth to specify the device only, i.e. "pci.0/lsi" will be
accepted too.

qdev_device_add() adds devices and accepts bus= parameters to find the
bus the device should be attached to.  Without bus= being specified it
takes the first bus it finds where the device can be attached to (i.e.
first pci bus for pci devices, ...).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>