Define the common `interface` for `qemuimgutil` and `nativeimgutil`

[AkihiroSuda]

https://github.com/lima-vm/lima/blob/master/pkg/qemu/imgutil/imgutil.go
https://github.com/lima-vm/lima/blob/master/pkg/nativeimgutil/nativeimgutil.go

These package should implement the common interface.

• pkg/imgutil: define the interface`
• pkg/imgutil/nativeimgutil: native implementation
• pkg/imgutil/qemuimgutil: qemu-img implementation

Probably there should be also pkg/imgutil/proxyimgutil (name is tentative) that attempts qemuimgutil first and then fall back to nativeimgutil?