implement vscsi-attach

tools/libxl/xl_cmdimpl.c implements checks for existing vscsi devices, prevent overwriting existing and similar policy

tools/libxl/libxl.c then works only upon fully initialized libxl_device_vscsi structure. Checks if backend vscsi host exists. If not, create backend and frontend for vscsi host and then add devices. Else add only devices but checks for existing ones which are skipped.

In case of creating new device, it waits until device is connected. Else it triggers device reconfiguration and skip connection check.