Add new function get_windows_drive_letters()

New function to get the drive letters has been assigned in windows guest.
Use this function to help assign a new drive letter to a new block device.

BZID: 1169678

Change from v1:
    Use regex to filter letters

Signed-off-by: Chengyou Liu <cheliu@redhat.com>
Merge pull request #1911 from cheliu/block_resize_drive_letter_fix

Add new function get_windows_drive_letters()