virttest: rename params in QemuImg.convert()

rename image_name_tag to convert_name_xx, because if image format not
change when convert, converted image will over write original;

Changes from V1:
    replace params.get() with params[] to avoid missing param errors.

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: new class for qemu-img function tests

Implement some basic operations for qemu-img function tests

Changes from V1:
    replace params.get() with params[] to avoid missing param errors.

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: add qemu_disk_img_convert tests

Implement to test qemu-img convert function; convert base image or
snapshot image from one format to another format, then check image content
not changed after image convert;

Changes from V1:
    replace params.get() with params[] to avoid missing param errors.

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: add qemu_disk_img_commit test

Implement new test to check qemu-img commit function;

Changes from V1:
    replace params.get() with params[] to avoid missing param errors.

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: add qemu_disk_img_rebase test

Implement new test to check qemu-img rebase function

Changes from V1:
    replace params.get() with params[] to avoid missing param errors.

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