virttest.qemu_storage: Update create() function when using qcow2 format

Some option only support qcow2 format, like: preallocation, encryption,
cluster_size.

Signed-off-by: Shuping Cui <scui@redhat.com>
virttest.qemu_storage: Fix a typo

We use 'encryption=on' for qcow2 format now.

Signed-off-by: Shuping Cui <scui@redhat.com>