add support for drive ids.

add support for drive ids.

-drive accepts the new id= now, allowing to explicitely name your
drives.  They will show up with that name in "info block" if specified,
otherwise the existing namimg scheme is used to autogenerate one.

There is also a new function to lookup drives by name.  Not used yet.
The plan is to link disk drivers and drives using the drive id instead
of passing around pointers to BlockDriveState.

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