Use KB as the unit for LVM PE size

LogicalVolume.Attach in bdev.py is calling "lvs" and specifying a unit of
Megabytes that is then converted to an integer resulting in the value 0 for
small sizes.

This patch makes Ganeti use KB for the unit instead of MB.

Fixes Issue 347.

Signed-off-by: Aaron Knister <aaron.knister@gmail.com>
Reviewed-by: Michele Tartara <mtartara@google.com>