Starting the containers of a multi-container container group in a single
confidential WCOW UVM concurrently reliably crashes the guest: the guest
resets and the GCS bridge drops ("bridge closed: use of closed network
connection"), and all containers end up Exited.

Root cause is concurrent container bring-up into one confidential UVM.
createContainer runs on its own goroutine per container, so two containers'
hcsoci.CreateContainer calls overlap. CreateContainer performs the container
bring-up: block-CIM mount, scratch SCSI attach, CombineLayers/hive-merge and
the guest container create. The host-side device hot-adds (uvm.modify ->
hcsSystem.Modify) go straight to the VM worker and do not travel over the GCS
bridge, so nothing serializes them; overlapping mount/device operations into
the confidential guest put it into a bad state and it resets.

Serialize the bring-up with a per-UVM lock held across CreateContainer, taken
only for confidential UVMs (HasConfidentialPolicy). This makes the container
starts effectively one-at-a-time into a given confidential UVM. createContainer
is shared with LCOW, hence the generic name and the confidential-only guard.

Validated on a confidential WCOW UVM (VBS): concurrent multi-container groups
that previously crashed every time (2x nanoserver, 2x mount-host,
1x mount-host + 2x nanoserver) now come up cleanly and repeatably; removing the
lock reproduces the crash on the same image.

Signed-off-by: Emanuel Oprea <2664342+EmanuelOprea@users.noreply.github.com>