libdm: Implement LoadTableAndActivate.

This change implements DeviceMapper::LoadTableAndActivate by serializing
the given DmTable and issuing DM_TABLE_LOAD and DM_DEV_SUSPEND ioctls.
In addition, this makes the CreateDevice() method private, and
introduces a separate method for creating a device and loading a table
as a single operation. This will obviate the need for separate inactive
device cleanup logic later.

Note that this change does not yet implement DmTable::Serialize().

Bug: 110035986
Test: N/A
Change-Id: Ic8affe591db4930ce672b00db989978b57ca8cbf