ANDROID: KVM: arm64: Factor out logic for setting SVE vector length at hyp

Factor out the logic for setting the SVE vector length at the
hypervisor in pKVM to the maximum hardware supported value when
saving and restoring host SVE state.

This shares common code, and elides an unused variable warning
when SVE is not configured.

Change-Id: Ibaa58e5bb44e4f85b9548a6abb498f0a769f6a4b
Signed-off-by: Fuad Tabba <tabba@google.com>
Bug: 267291591