tests/qtest/migration: Fix file migration offset check

tests/qtest/migration: Fix file migration offset check

When doing file migration, QEMU accepts an offset that should be
skipped when writing the migration stream to the file. The purpose of
the offset is to allow the management layer to put its own metadata at
the start of the file.

We have tests for this in migration-test, but only testing that the
migration stream starts at the correct offset and not that it actually
leaves the data intact. Unsurprisingly, there's been a bug in that
area that the tests didn't catch.

Fix the tests to write some data to the offset region and check that
it's actually there after the migration.

While here, switch to using g_get_file_contents() which is more
portable than mmap().

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>