libsnapshot: test: add changes to work with ublk enabled

These are changes to snapshot_test.cpp to work with ublk enabled
snapuserd. Changes are mostly straightforward, removing the assumption
that userspace snapshot devices are only dm-user.

Test: vts_libsnapshot_test

Bug: 414812023
Test: th
Change-Id: I28a41d0db275bbed023da8b0bcc41564d021e3a2
Signed-off-by: Sandeep Dhavale <dhavale@google.com>