virttest.libvirt_xml: Add API to check if given disk exists in domain

Signed-off-by: Yanbing Du <ydu@redhat.com>
virttest.qemu_storage: API to get qemu image format

Signed-off-by: Yanbing Du <ydu@redhat.com>
virttest.utils_test.libvirt: API to check block job

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1417 from autotest/1317

Remake of PR 1317