qemu.tests.cpu_hotplug: Call vm.hotplug_vcpu() to hotplug vcpu.

vm.hotplug_vcpu() will select default command for human/qmp monitor.

In some version, cpu_set have been removed from human monitor,
vm.hotplug() will throw TestNAError.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests: make qemu-img convert to cover more test scenarios

Test plan updated, we need to make qemu_img test option 'preallocation'
and 'cluster_size' together to avoid bug like:
qemu-img coredump when create image with cluster_size > 65536 and
option 'preallocation=metadata';

Signed-off-by: Xu Tian <xutian@redhat.com>