build: add make dist target (v2)

Let's stop screwing up releases by having a script do the work that Anthony's
fat fingers can't seem to get right.

Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>