blkfront: add heuristics to 'connect' which understand Linux devices lik...

...e 'xvda'

From the user's point of view the common case is:
* user writes an xl/xm config file which adds a disk with virtual
  device "xvda"
* [connect "xvda"] will connect to this block device
  (as will [connect "51712"] and [connect "/dev/xvda"])

connect will interpret the id as follows (first match wins)
1. id is a xenstore "bus slot number" (e.g. conventionally Linux
   major/minor numbers encoded as a single integer like 51712,
   but which can be any integer)
2. id is a "Linux device string" (with or without "/dev/" prefix)
   translated into a "bus slot number"

Signed-off-by: David Scott <dave.scott@eu.citrix.com>