virttest.qemu_storage: Refactoring and new compare_images method

Refactor qemu_storage.QemuImg() so that it's more logical
to query for available qemu-img commands. Also, introduce
a method .compare_images() method that will take 2 images
and compare them using the subcommand 'compare', if it is
available.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu.test: merge block_mirror test

* Fix incorrect command when run test with qmp monitor;
* Fix logical error that target image file should not
  be opened before the reopen step;
* Merge configuration file into drive_mirror.cfg
* Use by default upstream block mirror names rather than
  assuming RHEL block mirror names.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Xu Tian <xutian@redhat.com>