ramblocks: Make use of DeviceState pointer and BusInfo.get_dev_path

ramblocks: Make use of DeviceState pointer and BusInfo.get_dev_path

With these two pieces in place, we can start naming ramblocks.  When
the device is present and it lives on a bus that provides a device
path, we concatenate the path and the provided name.  Otherwise we
just use name.  The resulting id string must be unique.  For now we
assume an allocation for the same name and size is a device that has
been removed and reinserted and return the same block.  This will go
away once qemu_ram_free() is implemented.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>