Add new method as_any() to GuestAddressSpace

[jiangliu]

Add new method as_any() to GuestAddressSpace, which returns reference to a std::any::Any trait object. Then downcast_ref() could be used to get the concrete type of GuestAddressSpace trait object.

Signed-off-by: Liu Jiang gerry@linux.alibaba.com

Summary of the PR

Please summarize here why the changes in this PR are needed.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] Any newly added unsafe code is properly documented.

[lauralt]

@jiangliu can you fix the conflicts so we can merge the PR? Thanks.

[jiangliu]

@jiangliu can you fix the conflicts so we can merge the PR? Thanks.

@lauralt hi, I have updated the PR, thanks:)

[bonzini]

Sorry, I think the extra 'static bound is a problem, especially when the GuestAddressSpace is just &M. I don't think this pull request can be salvaged, you should just use external crates like as_any or downcast.

[bonzini]

(Did not mean to close, sorry).