vmctl: fix incorrect scaling when converting disk images.

vmctl: fix incorrect scaling when converting disk images.

ctl_convert still had old logic assuming disk sizes were in MiB and
not bytes, causing an attempt to create a new disk using the '-i'
argument to generate impossibly large output disk images (e.g. 1 MiB
ends up 1 TiB).

Reported by and diff from Jesper Wallin.