python-libguestfs should only be installed for libvirt/KVM.

And only when on a Redhat-based distribution

Change-Id: Ie99fd1ee54f5aaa639a3c5d8534315a4e2cac957
Ensure variables from stackrc are set for the fixup actions

In particular, VIRT_DRIVER needs to be set so we don't remove
python-lxml for xenserver-core based machines

Change-Id: Ic62567edab534f8af55f808ed645d8561e8ca59c
Adding support for xenserver local connections

Change-Id: I29ef3d74a30a3b8798c1669bba15205f50ab434e