Build: Build the thing

These are the changes needed to get 'rake -T' running in my
f18 + ruby 1.9.3 environment.
rbovirt: Preserve set headers

The headers passed into the http_headers method are merged last to
preserve the ones already set.

This preserves the 'detail=X' additions to the :accept header in
OVIRT::Client#vms, which in turn will save a few round trips for machine
details.
VM: Set interfaces and volumes, if available

Otherwise fallback to the given client as before.
client_spec: Unit test for Client

Added a new test to assert that Client#http_headers will preserve the
headers passed in. The http_headers method is marked private, and a full
on integration test is too cumbersome, so a unit test with the #send
method is used.