blktap2: human readable output for tapdisk2 creation problems

This patch fixes the "file object has no attribute find" failure
we've been seeing when starting blktap2 devices and adds more
meaningful error output to conditions where the tapdisk2 process is
unable to create a blktap2 device.

Signed-off-by: Dutch Meyer <dmeyer@cs.ubc.ca>

tools/python/xen/xend/server/BlktapController.py

@@ -32,7 +32,7 @@ def doexec(args, inputtext=None):
         proc.tochild.write(inputtext)
     stdout = proc.fromchild
     stderr = proc.childerr
-    rc = proc.poll()
+    rc = proc.wait()
     return (rc,stdout,stderr)
 
 def parseDeviceString(device):
@@ -131,7 +131,16 @@ class BlktapController(BlkifController):
         cmd = [ TAPDISK_BINARY, '-n', '%s:%s' % (params, file) ]
         (rc,stdout,stderr) = doexec(cmd)
 
-        minor, device, control = parseDeviceString(stdout)
+        if rc != 0:
+            err = stderr.read();
+            out = stdout.read();
+            stdout.close();
+            stderr.close();
+            raise Exception, 'Failed to create device.\n    stdout: %s\n    stderr: %s\nCheck that target \"%s\" exists and that blktap2 driver installed in dom0.' % (out.rstrip(), err.rstrip(), file);
+
+        minor, device, control = parseDeviceString(stdout.readline())
+        stdout.close();
+        stderr.close();
 
         #modify the configuration to attach as a vbd, now that the
         #device is configured.  Then continue to create the device