qemu.tests.qemu_img: Correct create command with -o option

if preallocated == metadata and cluster_size is not None, will get command like:
qemu-img create -f qcow2 -o prellaction=metadata -o cluster_size=xxx,
correct command should be: qemu-img create -f qcow2 -o prellaction=metadata,cluster_size=xxx

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