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blktap1 has been replaced by blktap2 that does not support "tap"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
Add notifiers to both attach and detach volumes

Implements: notifiers for attach and detach volumes
Fixes: bug #1191072
Change-Id: I84bc4d5eea6bfc9fe73ca09a5bec0e1e4d14ae02
Merge "Add notifiers to both attach and detach volumes"
Merge "Select disk driver for libvirt+Xen according to the Xen version"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp