About KVM screenshot

[Lei Yu]

This email is to introduce some progress and ask for comments about
the KVM screenshot function.

The whole KVM screenshot is implemented by the below pieces:
1. The vncsnapshot[1] is slightly modified for cross-compile, and a
bitbake recipe is created for it;
2. A service is created to host the DBus interface to provide the
"Trigger" and "Clear" methods for KVM screenshot;
3. OEM redfish actions are implemented to trigger and clear the KVM
screenshot, including the download of the picture;

Be noted that the function is currently experimental and not stable:
* vncsnapshot has some bug on skipping the black screen, so we have to
add a delay to capture the screen;
* When the BMC is in high load, it takes longer to open and capture
the KVM, so the above delay may not be enough.

If the work is got interested, we could try to upstream the changes, including:
* Submit a patch to meta-openembedded to add vncsnapshot recipe (with
patches for cross-compile)
* Add an interface to phosphor-dbus-interface to host the service interface;
* Add a new daemon (in a new repo or share some existing repo?)
* Submit patches to bmcweb about the OEM Redfish actions (for now it's
Bytedance OEM action)

Feedbacks and suggestions are welcome.

[1]: https://sourceforge.net/projects/vncsnapshot/files/vncsnapshot/1.2a/

-- 
BRs,
Lei YU