[Lucid-EC2 SRU] Pick/backport patches for blkfront

[Stefan Bader]

On Amazon EC2, some users experience problems when attaching and
detaching xen-blkfront (disk) devices repeatedly.
The following 4 patches (cherry-picks or simple backports) are
said to fix the issue.
Since this is affecting only the ec2 kernels and only xen guests,
testing is supposed to ensure a low regression potential.

NOTE: I already got this staged together with this cycles rebase
against master on my local tree. Just got a bit rusty on the
sequence of things. So I will be doing the commit part myself.
Just realized that this needs ACKs.

-Stefan