i386/sev: add support for confidential guest reset

When the KVM VM file descriptor changes as a part of the confidential guest
reset mechanism, it necessary to create a new confidential guest context and
re-encrypt the VM memory. This happens for SEV-ES and SEV-SNP virtual machines
as a part of SEV_LAUNCH_FINISH, SEV_SNP_LAUNCH_FINISH operations.

A new resettable interface for SEV module has been added. A new reset callback
for the reset 'exit' state has been implemented to perform the above operations
when the VM file descriptor has changed during VM reset.

Tracepoints has been added also for tracing purpose.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-23-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>