virttest.qemu_io: Add essential_option check in qemu_io

Add essential option check in qemu_io to ignore the wrong error
raised when the essential_option is empty.

change from v1:
  - fix the wrong judgement

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest.qemu_io: qemu_io.py use correct qemu_io link in qemu_io_cmd

Update the qemu_io link with the new interface in data_dir

Signed-off-by: Feng Yang <fyang@redhat.com>
Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest.qemu_storage: Add snapshot list cmd in kvm_storage

This functions is used to list all snapshots for given image.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest.qemu_storage: Add error.context in create() function

Add error.context in create() function to record the image create
command.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.qemu_storage: Support -t parameter in commit, rebase, convert function

Changes from v1:
Add cache_mode as a parameter of rebase, commit, convert function.

Signed-off-by: Feng Yang <fyang@redhat.com>