In order to support use cases such as migration, it may be
important in some situations to restrict the set of SVE vector
lengths available to the guest.  It can also be useful to observe
the behaviour of guest OSes with different vector lengths.

To enable testing and experimentation for such configurations, this
patch adds a command-line option to allow setting of the set of
vector lengths to be made available to the guest.

For now, the setting is global: no means is offered to configure
individual guest vcpus independently of each other.

By default all vector lengths that the host can support are given
to the guest, as before.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>