xen-blkback: refactor vbd remove/disconnect.

xen-blkback: refactor vbd remove/disconnect.

This patch refactor vbd remove/disconnect.
1. Add blkback shutdown watch for the remove/disconnect.
2. Don't disconnect backend when frontend state is XenbusStateClosing
   until frontend state changed to XenbusStateClosed.

Signed-off-by: Joe Jin <joe.jin@oracle.com>
Cc: Daniel Stodden <daniel.stodden@citrix.com>
Cc: Jens Axboe <jaxboe@fusionio.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Annie Li <annie.li@oracle.com>
Cc: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>