dmg: coding style and indentation cleanup

Clean up the mix of tabs and spaces, as well as the coding style
violations in block/dmg.c.  There are no semantic changes since this
patch simply reformats the code.

This patch is necessary before we can make meaningful changes to this
file, due to the inconsistent formatting and confusing indentation.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
