virt: force TCG with libguestfs unless KVM is enabled in libvirt

If the libvirt driver has not been configured to use KVM, then
the libguestfs module should be forced to use TCG. This is
particularly important when running Nova inside a VM, which
might claim to have VMX/SVM support when it is in fact broken.
This will avoid libguestfs hanging in such scenarios.

Resolves-bug: #1286256
Change-Id: I9316dcedd65244c60d468b270311f032b45b051f
Merge "virt: force TCG with libguestfs unless KVM is enabled in libvirt"