The current network namespace parameter nested in the JailerConfig
parameter requires specifying the entire jailer configuration, which is not
convenient and doesn't allow using the noop jailer with a network
namespace.
To overcome this limitation, add a new NetNS parameter to the CreateVM
request.
See also a similar change in firecracker-microvm/firecracker-go-sdk#155.
Signed-off-by: Georgiy Lebedev <lebedev.gk@phystech.edu>

In order to support firecracker snapshot-restore we need to make the
following changes:
* add a new CreateSnapshot request;
* add new parameters for snapshot loading to the CreateVM request
(following the firecracker Golang SDK design, snapshot loading is
essentially creating a VM with additional snapshot options).
When a VM is created by loading a snapshot, it's container snapshot drives
are already mounted, so the drive mount stub creation and mounting is
skipped.
Also add a NetNS (network namespace) parameter to the CreateVM request,
since the current NetNS parameter nested in the JailerConfig parameter
requires specifying the entire jailer configuration, which is not
convenient.
Signed-off-by: Georgiy Lebedev <lebedev.gk@phystech.edu>