block: vhdx - fix comment typos in header, fix incorrect struct fields

VHDXPage83Data and VHDXParentLocatorHeader both incorrectly had their
MSGUID fields set as arrays of 16.  This is incorrect (it stems from
an early version where those fields were uint_8 arrays).  Those fields
were, up to this patch, unused.

Also, there were a couple of typos and incorrect wording in comments,
and those have been fixed up as well.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>