OEcore/bitbake are moving to use the clearer ":" as an overrides
separator.

This is pass one of updating the meta-virt recipes to use that
syntax.

This has only been minimally build/runtime tested, more changes
will be required for missed overrides, or incorrect conversions

Note: A recent bitbake is required:

    commit 75fad23fc06c008a03414a1fc288a8614c6af9ca
    Author: Richard Purdie <richard.purdie@linuxfoundation.org>
    Date:   Sun Jul 18 12:59:15 2021 +0100

        bitbake: data_smart/parse: Allow ':' characters in variable/function names

        It is becomming increasingly clear we need to find a way to show what
        is/is not an override in our syntax. We need to do this in a way which
        is clear to users, readable and in a way we can transition to.

        The most effective way I've found to this is to use the ":" charater
        to directly replace "_" where an override is being specified. This
        includes "append", "prepend" and "re...