Bundle copy can be faster

[guillaumerose]

On my machine, the bundle copy from the cache to the machine directory takes around 6s with the Go code.
With cp it takes 4s.

Related code: https://github.com/code-ready/machine-driver-hyperkit/blob/master/pkg/hyperkit/copy.go#L8

[cfergeau]

The copy code is mostly a call to io.Copy(out, in), if there are improvements to be made, they may belong to this module. Or could it be a fsync/no fsync difference?
Btw, defer out.Close() is frowned upon as we could miss errors (https://www.joeshaw.org/dont-defer-close-on-writable-files/ )