KVM: arm64: Save/restore SVE state for nVHE

Implement the SVE save/restore for nVHE, following a similar
logic to that of the VHE implementation:

- the SVE state is switched on trap from EL1 to EL2

- no further changes to ZCR_EL2 occur as long as the guest isn't
  preempted or exit to userspace

- ZCR_EL2 is reset to its default value on the first SVE access from
  the host EL1, and ZCR_EL1 restored to the default guest value in
  vcpu_put()

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>