qemu.tests: Measure RHEL guest boot up time

Measure the boot up time of RHEL guest when it run level is 1.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Golita Yue<gyue@redhat.com>

get_image_filename have been moved to virt_storage.
And we have moved image related script to virt_storage.

Changes from v2:
Delete duplicated module
Changes from v1:
Import missing kvm_storage in live_snapshot_chain.py

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

All image related functions is moved to storage class. Update it
in boot_time.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>

backup image before boot_time test and restore image after it.

Delete image snapshot part code and simplify the code.

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

Update method for set and resotre run level. Also drop cache before
test the boot time.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>