Move the hypervisor attribute to the instances

This (big) patch moves the hypervisor type from the cluster to the
instance level; the cluster attribute remains as the default hypervisor,
and will be renamed accordingly in a next patch. The cluster also gains
the ‘enable_hypervisors’ attribute, and instances can be created with
any of the enabled ones (no provision yet for changing that attribute).

The many many changes in the rpc/backend layer are due to the fact that
all backend code read the hypervisor from the local copy of the config,
and now we have to send it (either in the instance object, or as a
separate parameter) for each function.

The node list by default will list the node free/total memory for the
default hypervisor, a new flag to it should exist to select another
hypervisor. Instance list has a new field, hypervisor, that shows the
instance hypervisor. Cluster verify runs for all enabled hypervisor
types.

The new FIXMEs are related to IAllocator, since now the node
total/free/used memory counts are wrong (we can't reliably compute the
free memory).

Reviewed-by: imsnah