mkvirtdisk fixes + macos support

[erichchan999]

• Added support for macOS. fdisk behaves differently there.
• Added check for dosfstools needed for mkfs.fat
• Specifies FAT12 for the filesystem in each partition, when previously it was using the default in mkfs.fat
• Now handles the problem of beginning sector being either 1 or 2048 depending on the size of the disk (a restriction only in the linux case).
• Added a check for minimum size in disk that would break either tool of fdisk or mkfs.fat.
• Silence dd stderr. It seems to direct logging there, causing the zig build system Building virtio example with zig #65 to report an error.

[Ivan-Velickovic]

Fantastic 🤌