virt: Fix a bug in RemoteFile init

If remote file doesn't exist, SCPTransferFailedError
will be catched, and then creat a new empty file om
local host.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add clean() func into RemoteFile class

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>