main: Clarify truncate behavior for event monitor file

Fix beta clippy issue:

error: file opened with `create`, but `truncate` behavior not defined
   --> src/main.rs:624:26
    |
624 |                         .create(true)
    |                          ^^^^^^^^^^^^- help: add: `.truncate(true)`
    |
    = help: if you intend to overwrite an existing file entirely, call `.truncate(true)`
    = help: if you instead know that you may want to keep some parts of the old file, call `.truncate(false)`
    = help: alternatively, use `.append(true)` to append to the file instead of overwriting it
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_open_options
    = note: `-D clippy::suspicious-open-options` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::suspicious_open_options)]`

Signed-off-by: Bo Chen <chen.bo@intel.com>

src/main.rs

@@ -622,6 +622,7 @@ fn start_vmm(cmd_arguments: ArgMatches) -> Result<Option<String>, Error> {
                     std::fs::OpenOptions::new()
                         .write(true)
                         .create(true)
+                        .truncate(true)
                         .open(parser.get("path").unwrap())
                         .map_err(Error::EventMonitorIo)?,
                 ))