Extract DRBD-related classes into block/drbd.py

This is in preparation to introducing support for DRBD8.4.
base.py had to be extracted as well in order to avoid cylic imports
between bdev.py and drbd.py. It now contains the BlockDev class and
utility functions needed by bdev.py and drbd.py.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>