Tools: hv: fix file overwriting of hv_fcopy_daemon

hv_fcopy_daemon fails to overwrite a file if the target file already
exits.

Add O_TRUNC flag on opening.

Signed-off-by: Yue Zhang <yuezha@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>