virttest.qemu_vm: Check migration_exec_cmd_src before accessing it

Signed-off-by: Qingtang Zhou <zhouqt@gmail.com>
qemu.tests: fix login test fail when need_install is yes

VM paused after install finished because of option '-no-shutdown', and
env.get_vm() got a paused VM, so case will fail at wait_for_login() when
need_install is 'yes';

Signed-off-by: Xu Tian <xutian@redhat.com>
virttest: Added check_image to yes in checkable formats

and modify the qemu_storage.py to expand to support check qed qcow2v3
and increase the verbose to let people know the format in debug file.

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
shared.cfg: fix a typo in guest-hw

fix a typo introduced by me in 651,
the "image_format" param should still be qcow2 for qcow2v3.
I was renaming the qcow3 to qcow2_v3 in 630, but I mistakenly set the branch to master when
opening the pullreq, then reopen it on 651 to branch next, but that time I mistakenly modified the 'image_format'

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>