SFTP read/write calls ignore bytes written return value

[Sploder12]

Describe the bug
When handling SFTP read/write Multipass makes the assumption that the provided buffer is always filled, so the bytes written return value is ignored. This can cause issues like #3892 since this not a safe assumption. Fixing this would also make Multipass more future-proof against libssh changing the implementation of these functions.

To Reproduce

1. Create an instance foo using Ubuntu 20.04
2. Start the instance
3. multipass transfer a file larger than 32KiB into foo
4. Observe the resulting file is missing data

Expected behavior
The transferred filed is identical to the original. (Bytes written is handled properly)

Additional info

• OS: Ubuntu 24.04
• CPU architecture: x86_64
• multipass get local.driver: qemu

Additional context
Reproducible from 5ee1dab onwards, but will not be if #3902 gets merged.
This issue isn't reproducible with 24.04 VMs, unclear why.