mlog: add a new log file with the setup logs

[dabrain34]

Add meson-setup.txt to keep the configure log
out of the debug logs.

[dabrain34]

@xclaesse WDYT ?

[bonzini]

I looked at the patch and I have no idea what it does. Your PR description should at least answer questions such as

• What commands should be invoked to notice the usefulness of the new log
• What kind of message is hard to find in meson-log.txt

[xclaesse]

What it does is saving in a file logs as they are printed on the terminal. meson-log.txt often contains too many info which makes it hard to read and logs printed on the terminal can be lost. It creates builddir/meson-logs/meson-setup.txt file.

@dabrain34 a releases notes snippet would help clarify what this does, otherwise nobody will notice this.

[dabrain34]

Added a snippet to release notes

[bonzini]

contains the configure logs displayed on stdout during the meson setup stage.

Yes this is exactly what I wanted to know