block/vhdx: check error return of bdrv_getlength()

Calls to bdrv_getlength() were not checking for error.  In vhdx.c, this
can lead to truncating an image file, so it is a definite bug.  In
vhdx-log.c, the path for improper behavior is less clear, but it is best
to check in any case.

Some minor code movement of the log_guid intialization, as well.

Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
