Abstract Linux node information in hv_base

Currently both hv_fake and hv_kvm implement practically identical code
to get the node information. Since future container-like hypervisors
will also need this functionality, this patch moves it into the base
class (as a separate function) which can then be called from classes
which need this info.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>