xen: add implementations of xen-block connect and disconnect functions...

xen: add implementations of xen-block connect and disconnect functions...

...and wire in the dataplane.

This patch adds the remaining code to make the xen-block XenDevice
functional. The parameters that a block frontend expects to find are
populated in the backend xenstore area, and the 'ring-ref' and
'event-channel' values specified in the frontend xenstore area are
mapped/bound and used to set up the dataplane.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
v3:
 - Add missing return statement to xen_block_realize()
 - Set device name to 'vbd' now the backend is functional
 - Add an unplug handler
 - Move backend watch code from subsequent patch since it's now needed
   to handle unplug

v2:
 - Tidy up header inclusions
 - Stop leaking ring_ref on error
 - Auto-create drive for CDRom devices