qemu: fix switchover-ack regression for old qemu

qemu: fix switchover-ack regression for old qemu

When enabling switchover-ack on qemu from libvirt, the .party value
was set to both source and target; however, qemuMigrationParamsCheck()
only takes that into account to validate that the remote side of the
migration supports the flag if it is marked optional or auto/always on.

In the case of switchover-ack, when enabled on only the dst and not
the src, the migration will fail if the src qemu does not support
switchover-ack, as the dst qemu will issue a switchover-ack msg:
qemu/migration/savevm.c ->
  loadvm_process_command ->
    migrate_send_rp_switchover_ack(mis) ->
      migrate_send_rp_message(mis, MIG_RP_MSG_SWITCHOVER_ACK, 0, NULL)

Since the src qemu doesn't understand messages with header_type ==
MIG_RP_MSG_SWITCHOVER_ACK, qemu will kill the migration with error:
  qemu-kvm: RP: Received invalid message 0x0007 length 0x0000
  qemu-kvm: Unable to write to socket: Bad file descriptor

Looking at the original commit [1] for optional migration capabilities,
it seems that the spirit of optional handling was to enhance a given
existing capability where possible. Given that switchover-ack
exclusively depends on return-path, adding it as optional to that cap
feels right.

[1] 61e34b08568 ("qemu: Add support for optional migration capabilities")

Fixes: 1cc7737f69e ("qemu: add support for qemu switchover-ack")
Signed-off-by: Jon Kohler <jon@nutanix.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Avihai Horon <avihaih@nvidia.com>
Cc: Jiri Denemark <jdenemar@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: YangHang Liu <yanghliu@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>