sev: Provide sev_features flags from IGVM VMSA to KVM_SEV_INIT2

sev: Provide sev_features flags from IGVM VMSA to KVM_SEV_INIT2

IGVM files can contain an initial VMSA that should be applied to each
vcpu as part of the initial guest state. The sev_features flags are
provided as part of the VMSA structure. However, KVM only allows
sev_features to be set during initialization and not as the guest is
being prepared for launch.

This patch queries KVM for the supported set of sev_features flags and
processes the VP context entries in the IGVM file during kvm_init to
determine any sev_features flags set in the IGVM file. These are then
provided in the call to KVM_SEV_INIT2 to ensure the guest state
matches that specified in the IGVM file.

The igvm process() function is modified to allow a partial processing
of the file during initialization, with only the IGVM_VHT_VP_CONTEXT
fields being processed. This means the function is called twice,
firstly to extract the sev_features then secondly to actually
configure the guest.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/b2f986aae04e1da2aee530c9be22a54c0c59a560.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>