add utility method to read ext4 superblock from a VHD
add utility method to read ext4 superblock from a VHD

The change enables getting accurate information about ext4 fs on
a given VHD, rather than doing os.Stat or temp mounting the VHD.

Signed-off-by: Maksim An <b28a1069b720cf3786e288578c9a7f6f01d55828@microsoft.com>

Microsoft/hcsshim