qemu.tests: Add new subtest to qemu_img

Signed-off-by: Feng Yang <fyang@redhat.com>
KVM-Test: Boot/shutdown rebased image
Signed-off-by: Suqin Huang <shuang@redhat.com>
KVM Test: check the image after image creation.
KVM test: qemu_img: Pre-allocated metadata qcow2 format support
KVM Test: Raw device support for qemu_img test
KVM test: qemu_img add the qcow2 over LVM checking
Signed-off-by: Jason Wang <jasowang@redhat.com>
KVM Test: Add cluster_size_2048 test case.
Bug fix and add error.context in qemu_img.py script
Fix bug that guest start twice.
Signed-off-by: Feng Yang <fyang@redhat.com>