Add create option 'format', with enums:
    monolithicSparse
    monolithicFlat
    twoGbMaxExtentSparse
    twoGbMaxExtentFlat
Each creates a subformat image file. The default is monolithicSparse.

Signed-off-by: Fam Zheng <famcool@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>