disk-create: Fix this API so it works correctly with block devices (RHBZ#1088262).

When you call guestfs_disk_create on a block device with format=raw
then it will try to discard the blocks on the device.
builder: Suggest using virt-install --os-variant option in virt-builder man page.
builder: If using -v (verbose), ensure #disk_create runs verbose too.
sparsify: If using -v (verbose), ensure #disk_create runs verbose too.