pkg/image/nbdkit: Enable recommended nbdkit VDDK debugging options

VDDK datapath debugging logs every VDDK read and write which is noisy
and unnecessary.

Enabling VDDK stats gives useful information about how long each VDDK
call took, for virtually no overhead.  This information is printed to
stderr when nbdkit closes the connection.

This matches the upstream recommendations here, and also the flags
used by virt-v2v:

https://libguestfs.org/nbdkit-vddk-plugin.1.html#Troubleshooting-performance-problems

Signed-off-by: Richard W.M. Jones <[email protected]>

pkg/image/nbdkit.go

@@ -145,6 +145,8 @@ func NewNbdkitVddk(nbdkitPidFile, socket, server, username, password, thumbprint
 	}
 	pluginArgs = append(pluginArgs, "--verbose")
 	pluginArgs = append(pluginArgs, "-D", "nbdkit.backend.datapath=0")
+	pluginArgs = append(pluginArgs, "-D", "vddk.datapath=0")
+	pluginArgs = append(pluginArgs, "-D", "vddk.stats=1")
 	p := getVddkPluginPath()
 	n := &Nbdkit{
 		NbdPidFile: nbdkitPidFile,