[XENAPI] Update debugging scripts to support vbd_list

Add function to list all vbds attached to a VM.
Update VM config builder to specify the default as 'linux'

Signed-off-by: Alastair Tse <atse@xensource.com>

tools/python/scripts/xapi.domcfg.py

@@ -26,7 +26,7 @@ platform_serial =  ''
 platform_localtime =  False
 platform_clock_offset =  False
 platform_enable_audio =  False
-builder =  ''
+builder =  'linux'
 boot_method =  '' # this will remove the kernel/initrd ??
 kernel_kernel =  '/boot/vmlinuz-2.6.16.29-xen'
 kernel_initrd =  '/root/initrd-2.6.16.29-xen.img'