[Libvir] Allow default hypervisor URI to be set via ENV variable

[Daniel P. Berrange]

When using a non-Xen backend with virsh, it gets pretty tedious always 
passing '--default test:///some/big/path' - particularly if you're never
using Xen on that machine. Thus I figured it would be useful to allow
the default Xen URI to be overridden via an environment variable. 

  [berrange at localhost src]$ unset VIRSH_DEFAULT_CONNECT_URI
  [berrange at localhost src]$ ./virsh list
  failed to find libvirt_proxy
  libvir: error : no support for hypervisor
  lt-virsh: error: failed to connect to the hypervisor
  [berrange at localhost src]$ export VIRSH_DEFAULT_CONNECT_URI=test:///default
  [berrange at localhost src]$ ./virsh list
   Id Name                 State
  ----------------------------------
    0 Domain-0             running 

This will be even more useful when other real hypervisors are available
for libvirt. 

The patch also removes the assumption in virsh that there is >= 1 guest
domains - sure Xen always has a Domain-0, but its not guarenteed that
other non-Xen HVs have a permanent Domain-0

Regards,
Dan.
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