fs: satisfy blkback with an attached but not activated disk
fs: satisfy blkback with an attached but not activated disk

I believe blkback needs to know the size of the disk in sectors
before completing the hotplug action, otherwise it immediately
unplugs. Therefore supply blkback with a tapdisk containing a
correctly-sized disk, which we immediately "pause". In activate
we "unpause" with the real disk, once it is ready.

xapi-project/message-switch,djs55/message-switch,xapi-project/message-switch,djs55/message-switch-1,djs55/message-switch-1,djs55/message-switch-1,djs55/message-switch,djs55/message-switch,xapi-project/message-switch