tests: drivers: virtualization: move ivshmem test to a new folder

Move the existing ivshmem test to a folder that explicitly identifies
its target: ivshmem-plain. The existing test is not compatible with
ivshmem-doorbell.

ivshmem-doorbell has additional requirements compared to -plain
(ivshmem-server, notification handling), so it is reasonable that
another test is required for it. This commits prepares the way for the new
test.

No funcional change intended.

Signed-off-by: Rodrigo Cataldo <rodrigo.cataldo@huawei.com>
Co-authored-by: Henri Xavier <datacomos@huawei.com>