qemu.tests.cfg: Update some config for block_resize

Since _equal suffix not support now,
so update some config for windows guest:
    1. Change guest_prepare_cmd_equal option to guest_prepare_cmd.
    2. Add rescan step for disk_update_cmd.

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #96 from spcui/block_resize_fix

qemu.tests.cfg: Update some config for block_resize