KVM: arm64: Enumerate SVE register indices for KVM_GET_REG_LIST

This patch includes the SVE register IDs in the list returned by
KVM_GET_REG_LIST, as appropriate.

On a non-SVE-enabled vcpu, no new IDs are added.

On an SVE-enabled vcpu, IDs for the FPSIMD V-registers are removed
from the list, since userspace is required to access the Z-
registers instead to access their context.  For the variably-sized
SVE registers, the appropriate set of slice IDs are enumerated,
depending on the maximum vector length for the vcpu.

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