esx: Restrict vpx:// to handle a single host in a vCenter

esx: Restrict vpx:// to handle a single host in a vCenter

Now a vpx:// connection has an explicitly specified host. This
allows to enabled several functions for a vpx:// connection
again, like host UUID, hostname, general node info, max vCPU
count, free memory, migration and defining new domains.

Lookup datacenter, compute resource, resource pool and host
system once and cache them. This simplifies the rest of the
code and reduces overall HTTP(S) traffic a bit.

esx:// and vpx:// can be mixed freely for a migration.

Ensure that migration source and destination refer to the
same vCenter. Also directly encode the resource pool and
host system object IDs into the migration URI in the prepare
function. Then directly build managed object references in
the perform function instead of re-looking up already known
information.