[v1.12] Expose memory mapping; Make memfile dump optional

[ValentaTomas]

To gain more control over memfile handling during snapshotting, we modify the snapshot logic to optionally skip dumping the memfile.

Also, it adds info about the memory regions when requesting instance info, so we can copy the memory directly.

This is fully compatible with FC v1.12.

This PR is for viewing code changes only and won't be merged.

[ValentaTomas]

We might also make the snapfile optional/return it as content instead.

[tomassrnka]

AI helped review:

• /memory full scan faults pages in — Vm::get_memory_info (src/vmm/src/vstate/vm.rs) walks every page and memcmps each “resident” page; if mincore fails it assumes all pages are resident and memcmps them anyway. This can fault nonresident/ballooned pages back in, pull gigabytes into memory, and block the VMM lock for the duration. At minimum, skip memcmp unless mincore says resident, propagate mincore errors, and consider chunking/limits or scanning outside the VMM lock.

• Long-held VMM lock during scan — RuntimeApiController::GetMemory holds the global VMM mutex while calling get_memory_info, so the entire page scan runs under the lock. On large guests this can block other API operations and VM work. Collect mappings under the lock and perform the scan without it, or otherwise limit scan time.

[tomassrnka]

discussed in IRL, looks good to me