[virt-tools-list] [libosinfo v3] API to query required user avatar format

[Zeeshan Ali (Khattak)]

This version turns:

* mime-type property into a list (of strings).
* Make mime-type mandatory, i-e if avatar-format is specified by a script, it
  must also specify allowed mime-types.
* InstallScript:avatar-format prop declaration to use G_PARAM_STATIC_STRINGS