Raises an explicit error message when the domain config file specifies

an incorrect path to a file-backed VBD (disk image) or the file-backed
VBD does not exist.

Without this patch, if someone mis-types the path to the disk image,
he/she would have no indication as to what's wrong when the domain
fails to get created.

Signed-off-by: Khoa Huynh <khoa@us.ibm.com>

tools/python/xen/xend/image.py

@@ -274,6 +274,10 @@ class HVMImageHandler(ImageHandler):
                 uname = sxp.child_value(info, 'uname')
                 typedev = sxp.child_value(info, 'dev')
                 (_, vbdparam) = string.split(uname, ':', 1)
+
+                if 'file:' in uname and not os.path.isfile(vbdparam):
+                   raise VmError('Disk image does not exist: %s' % vbdparam)
+
                 if 'ioemu:' in typedev:
                     (emtype, vbddev) = string.split(typedev, ':', 1)
                 else: