set_log_base: use MmapRegion::bitmap() directly

[bonzini]

Summary of the PR

For use in QEMU, I would like GuestMemoryRegion to return a BitmapSlice instead of a &Bitmap (rust-vmm/vm-memory#324). This adds some flexibility that QEMU needs in order to support a single global dirty bitmap that is sliced by the various GuestMemoryRegions.

However, this removes access to the BitmapReplace trait, because it is of course not possible to replace a slice of the bitmap only. Fortunately, vhost is built around the GM<> type alias, which has a pluggable bitmap type but hardcodes the backend:

type GM<B> = GuestMemoryAtomic<GuestMemoryMmap<B>>;

and therefore region is known to be a GuestRegionMmap. Adding a single dereference of the GuestRegionMmap returns the MmapRegion to which the bitmap is attached, thus calling MmapRegion::bitmap() instead of ::bitmap().

The change works either with or without the changes in vm-memory.

Requirements

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