KVM: arm64: Implement lazy vCPU state sync for non-protected guests

pKVM copies a non-protected guest's register context between the host
and the hypervisor on every world switch, even when the host never
inspects it. Defer the copy: on entry, flush the host context into the
hyp vCPU only when the host marked it dirty (PKVM_HOST_STATE_DIRTY); on
exit, leave it in the hyp vCPU and copy it back only when the host needs
it, via a __pkvm_vcpu_sync_state hypercall or at vcpu put. A protected
guest's context is copied as before, since lazy sync only helps where
the host is trusted to see the guest's registers.

PC and PSTATE are the exception, copied back on every exit: the kvm_exit
tracepoint reports the guest's real exit PC, and
vcpu_mode_is_bad_32bit(), the SError-masking checks and the host
software-step state machine all read PSTATE before the next on-demand
sync. Without it an ARM_EXCEPTION_IRQ exit syncs nothing, so
flush_debug_state() pushes a stale PSTATE.SS back on the next entry and
single-stepping runs an extra instruction before the step is reported.

The host needs the full context when it is about to read it (trap
handling) or write it (the SError injection that writes ESR_EL1). Sync
both from handle_exit_early(), which runs non-preemptible so the loaded
hyp vCPU is stable without a preempt guard.

Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260729131823.2021516-9-fuad.tabba@linux.dev
Signed-off-by: Oliver Upton <oupton@kernel.org>

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