Select disk driver for libvirt+Xen according to the Xen version

There are two versions of the blktap [1] driver available under Xen.
blktap1 has been replaced by blktap2 that does not support "tap"
devices but "tap2", therefore we should use "tap2" devices by default.

Fixes bug 1188517
Fixes bug 1084618
DocImpact

Change-Id: I8b46885b0909de7cb51fd2d0f6ba2e9bcbf33cc6
Merge "Select disk driver for libvirt+Xen according to the Xen version"