xencommons: Attempt to load blktap2 driver

xencommons: Attempt to load blktap2 driver

Older kernels, such as those found in Debian Squeeze:
* Have bugs in handling of AIO into foreign pages
* Have blktap modules, which will cause qemu not to use AIO, but
  which are not loaded on boot.

Attempt to load blktap in xencommons, to make sure modern qemu's which
use AIO will work properly on those kernels.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Prefer to load blktap2 if it exists. This is the name of the driver in
classic-Xen ports, while in mainline kernels the driver is called just
blktap.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>