import_raw_vdi: Pass Content-Length to vhd-tool

This passes the `Content-Length` header to `vhd-tool serve` when using
raw non-chunked imports. This way clients don't need to manually pad out
the HTTP PUT with zeros, to fill it up to the next 2MB boundary.
Merge pull request #2063 from chengsun/fix-import-raw-vdi

import_raw_vdi: Pass Content-Length to vhd-tool