create_dynamic_disk() takes a buffer holding the footer as first
argument.  It writes out the footer (512 bytes), then reuses the
buffer to initialize and write out the dynamic header (1024 bytes).

Works, because the caller passes a buffer that is large enough for
both purposes.  I hate that.

Use a separate buffer for the dynamic header, and adjust the caller's
buffer.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201217162003.1102738-4-armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>