xen-blkfront: save uncompleted reqs in blkfront_resume()

xen-blkfront: save uncompleted reqs in blkfront_resume()

Uncompleted reqs used to be 'saved and resubmitted' in blkfront_recover() during
migration, but that's too later after multi-queue introduced.

After a migrate to another host (which may not have multiqueue support), the
number of rings (block hardware queues) may be changed and the ring and shadow
structure will also be reallocated.
So that blkfront_recover() can't 'save and resubmit' the real uncompleted reqs
because shadow structure has been reallocated.

This patch fixes this issue by moving the 'save' logic out of blkfront_recover()
to earlier place:blkfront_resume().

Orabug: 23340426
Signed-off-by: Bob Liu <bob.liu@oracle.com>