libvirt: Make `fakelibvirt.libvirtError` match

When test code uses `libvirtError`, it may refer to
`libvirt.libvirtError` or `fakelibvirt.libvirtError` depending on
whether `libvirt-python` is installed.

The existing code had incompatible `__init__` singatures between the
two, meaning that a test could pass locally but fail when run on
Jenkins.

The solution is to make `fakelibvirt.libvirtError` match libvirt 1.2.3
exactly.

A convenience function, `make_libvirtError`, is introduced so that
exceptions can still be created in one shot.

Change-Id: I5aaecc57504c4aea3c049610ef188978c25cfdc4
Closes-Bug: 1305186
Merge "libvirt: Make `fakelibvirt.libvirtError` match"