xen_disk: backport from upstream qemu

xen_disk: backport from upstream qemu

xen_disk is a pure userspace blkback implementation that can be used to
provided a disk backend called qdisk.
It is particularly useful with a dom0 kernel that doesn't have blktap2
(Linux 2.6.37).

[ This is a cherry pick of git commit
  62d23efac8905a46277f666c909e826f91c12aa1
aka git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7223
  c046a42c-6fe2-441c-8c8c-71466251a162
originally from Gerd Hoffman and Anthony Ligouri, edited
by Stefano to fit into current qemu-xen-unstable. -iwj ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>