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 <[email protected]>
Reviewed-by: Anthony Perard <[email protected]>
Signed-off-by: Anthony PERARD <[email protected]>