selftests/vsock: add namespace helpers to vmtest.sh

selftests/vsock: add namespace helpers to vmtest.sh

Add functions for initializing namespaces with the different vsock NS
modes. Callers can use add_namespaces() and del_namespaces() to create
namespaces global0, global1, local0, and local1.

The add_namespaces() function initializes global0, local0, etc... with
their respective vsock NS mode by toggling child_ns_mode before creating
the namespace.

Remove namespaces upon exiting the program in cleanup(). This is
unlikely to be needed for a healthy run, but it is useful for tests that
are manually killed mid-test.

This patch is in preparation for later namespace tests.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260121-vsock-vmtest-v16-5-2859a7512097@meta.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>