Detect and convert between endianess of guest

Previously, virtiofsd would simply error out when running a guest of a different endianess as it would receive a header for init that would include a size that is way bigger than it needs to be, and way bigger than we allocate. This makes for a good endianess test.

Now, we guess guest endianess using this size check, as the first header we receive will be < 100 bytes to allocate. Knowing this, we can detect endianess with this size check.

This also wraps all fuse structures with a GuestEndian type that uses a global to decide if they swap their bytes or not.

Resloves #145

Summary by Sourcery

Add support for detecting and converting between guest and host endianness in virtiofsd

New Features:

• Implement endianness detection for guests with different byte order
• Add support for converting FUSE structures between guest and host endianness

Bug Fixes:

• Resolve issue with guests of different endianness causing initialization errors

Enhancements:

• Wrap FUSE structures with a GuestEndian type to handle byte swapping
• Modify type conversions to support endianness conversion