create_config: separate section for qemu_*dir variables (v2)

create_config: separate section for qemu_*dir variables (v2)

The generic *dir section will eventually go away and be replaced with
qemu_* section. By now, both sections will be kept, while the variables
get renamed on config-host.mak.

With this patch, a XXXdir variable will become a CONFIG_QEMU_XXXDIR
define, and a qemu_XXXdir variable will become CONFIG_QEMU_XXXDIR as
well (instead of becoming a CONFIG_QEMU_QEMU_XXXDIR define).

Changes v1 -> v2:
 - Rebase on top of newer qemu.git changes, that changed
   "tr '[:lower:]' '[:upper:]'" to "LC_ALL=C tr '[a-z]' '[A-Z]'".

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>