Use new functions for windows guest

Use the new functions to avoid simple mistake like useless drive letter
Includes get_windows_drive_letters() and format_guest_disk()

Change from v1:
    1. When all the 26 letters been assigned, raise a error.
    2. If the last letter is 'Z', find the unassigned letter from 'A'

Signed-off-by: Chengyou Liu <cheliu@redhat.com>