libdm: suspend and resume devices

In some cases it would be required to suspend the device to succesfully
complete some operations. An example is the suspension of the origin
device that is necessary to avoid data corruption when merging a
snapshot.

Introduce suspend and resume ioctls in libdm.

Bug: 137759376
Test: libdm_test
Change-Id: Id2ff34e930a8b32e570cb9f49da9cc3f65cb499c
Signed-off-by: Alessio Balsini <balsini@google.com>