ANDROID: KVM: arm64: Use active guest SVE vector length on save/restore

On saving/restoring the guest SVE state, use the guest's current
active vector length. This reduces the amount of saving/restoring
for the cases where the maximum size isn't used. Moreover, it
fixes a bug where the ZCR_EL2 value wasn't being set when saving
the guest state, potentially corrupting it.

Bug: 278749606
Change-Id: If59ca3ee190c7a151f66b64deb6524a9eed00df5
Signed-off-by: Fuad Tabba <tabba@google.com>