xen 3.0.4 blktap:aio disks don't work with pygrub

[John Byrne <john.l.byrne@hp.com>]

Changeset 13112 (13134 in xen-unstable) added after the branch provoked 
the problem by causing some code that was previously dead in 
_configureBootloader() in XendDomainInfo.py. The code to mount the image 
loses the information that the tap driver is "aio" is lost and when 
device_add() in XendConfig.py, the code assumes the disk is qcow and 
things fail. (I assume things work fine for qcow disks.) If I change the 
"qcow" in device_add() to "aio", images boot; so I think the only thing 
needed to fix the problem is to get the information to device_add() and 
to generate the correct string.

The tap-driver information can be easily passed down in the xenapi_vbd, 
but I cannot determine what the proper key should be and I probably 
shouldn't just make it up since it is a XenAPI object and I probably 
don't understand some of the subtleties of adding it. I'd be happy to 
try with a little advice, though.

John Byrne