KVM: arm64/sve: Report and enable SVE API extensions for userspace

This patch adds the necessary API extensions to allow userspace to
detect SVE support for guests and enable it.

A new capability KVM_CAP_ARM_SVE is defined to allow userspace to
detect the availability of the KVM SVE API extensions in the usual
way.

Userspace needs to enable SVE explicitly per vcpu and configure the
set of SVE vector lengths available to the guest before the vcpu is
allowed to run.  For these purposes, a new arm64-specific vcpu
ioctl KVM_ARM_SVE_CONFIG is added, with the following subcommands
(in rough order of expected use):

KVM_ARM_SVE_CONFIG_QUERY: report the set of vector lengths
    supported by this host.

    The resulting set can be supplied directly to
    KVM_ARM_SVE_CONFIG_SET in order to obtain the maximal possible
    set, or used to inform userspace's decision on the appropriate
    set of vector lengths (possibly taking into account the
    configuration of other nodes in the cluster so that the VM can
    migrate freely).

KVM_ARM_SVE_CONFIG_SET: enable SVE for this vcpu and configure the
    set of vector lengths it offers to the guest.

    This can only be done once, before the vcpu is run.

KVM_ARM_SVE_CONFIG_GET: report the set of vector lengths available
    to the guest on this vcpu (for use when snapshotting or
    migrating a VM).

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