Added a global libvirt provider option ip_command to specify the ip_command

Also more robust handling of connection error when the libvirt connection fails

lib/fog/compute/libvirt.rb

@@ -30,18 +30,22 @@ module Fog
 
         attr_reader :connection
         attr_reader :uri
+        attr_reader :ip_command
 
 
-        # f=Fog::Compute.new(:provider => "Libvirt", :libvirt_uri => "qemu+ssh://patrick.debois@juno/system")
-        
         def initialize(options={})
           @uri = ::Fog::Compute::LibvirtUtil::URI.new(enhance_uri(options[:libvirt_uri]))
+          @ip_command = options[:libvirt_ip_command]
 
           # libvirt is part of the gem => ruby-libvirt
           require 'libvirt'
 
-          
+          begin
             @connection = ::Libvirt::open(@uri.uri)
+          rescue ::Libvirt::ConnectionError
+            raise Fog::Errors::Error.new("Error making a connection to libvirt URI #{@uri.uri}:\n#{$!}")
+          end
+
         end
 
         def enhance_uri(uri)

lib/fog/compute/models/libvirt/server.rb

@@ -247,9 +247,6 @@ module Fog
           raw.xml_desc
         end
 
-        ##Note this requires arpwatch to be running
-        ##and chmod o+x /var/lib/arpwatch
-
         # This retrieves the ip address of the mac address
         # It returns an array of public and private ip addresses
         # Currently only one ip address is returned, but in the future this could be multiple
@@ -259,7 +256,8 @@ module Fog
           mac=self.mac
 
           # Check if another ip_command string was provided
-          ip_command=options[:ip_command].nil? ? "grep #{mac} /var/log/arpwatch.log |cut -d ':' -f 4-| cut -d ' ' -f 4" : options[:ip_command]
+          ip_command_global=@connection.ip_command.nil? ? "grep #{mac} /var/log/arpwatch.log |cut -d ':' -f 4-| cut -d ' ' -f 4" : @connection.ip_command
+          ip_command=options[:ip_command].nil? ? ip_command_global : options[:ip_command]
 
           ip_address=nil