Fix Command::DmaMap

[blitz]

Summary of the PR

This PR fixes handling the DmaMap command. There are two issues:

1. When a DmaMap message arrives with one file descriptor, the backend still receives None in its dma_map call, because for fds.len() == 1 the code passes None instead of fds[0].
2. If the callee wants to hang on to the File object it gets via backend.dma_map(), they have a hard time, because we currently only pass a &File. Change this to File to give the callee more options to hang on to the File without resorting to unsafe libc::dup().

Technically only 1 is strictly necessary to get DMA mapping working:

2025-04-23T15:50:37.487297Z  INFO usbvfiod: dma_map flags = READ_ONLY | WRITE_ONLY | READ_WRITE offset = 0 address = 4294967296 size = 1073741824 fd = Some(File { fd: 6, path: "/memfd:ch_ram (deleted)", read: true, write: true })

Requirements

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

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[rbradford]

@likebreath Please review

[snue]

The API change to drop the reference and use Option<File> looks like a good simplification to me.

Thanks for fixing the off by one.

[likebreath]

@blitz Thanks for the contributions. Changes are looking good to me.

Would you please update the commit message with prefix server instead of vfio_user? This would be very helpful to clarify the scope of changes, say consumers of client APIs are not impacted (such as Cloud Hypervisor).

[blitz]

Would you please update the commit message with prefix server instead of vfio_user? This would be very helpful to clarify the scope of changes, say consumers of client APIs are not impacted (such as Cloud Hypervisor).

Good point. Done!

[rbradford]

Thanks! Still lgtm.