* fix(nbd-client): handle unreachable VM through nbd
when the user select nbd but don't have any way to reach
the storage/host of the VM (for example no NBD enabled network,
or a NBD enabled network unreachable by xo/proxy it was timing out
and blocking the start of the backup
This error was blocking the fallback to export stream mechanism
* fix(xapi): respect the config file setting for preferNbd
* fix(xapi): don't close export stream if nbd failed to connect
* fix(xapi):respect vhd/qcow2 when faling back to a full if delta exports fails