conf: Add tdx as launch security type

conf: Add tdx as launch security type

When 'tdx' is used, the VM will be launched with Intel TDX feature enabled.
TDX feature supports running encrypted VM (Trust Domain, TD) under the
control of KVM. A TD runs in a CPU model which protects the confidentiality
of its memory and its CPU state from other software.

There are four optional child elements. Element policy is 64bit hex, bit 0
is set to enable TDX debug, bit 28 is set to enable sept-ve-disable, other
bits are reserved currently. When policy isn't specified, QEMU will use its
own default value 0x10000000. mrConfigId, mrOwner and mrOwnerConfig are
base64 encoded SHA384 digest string.

For example:

 <launchSecurity type='tdx'>
   <policy>0x10000001</policy>
   <mrConfigId>xxx</mrConfigId>
   <mrOwner>xxx</mrOwner>
   <mrOwnerConfig>xxx</mrOwnerConfig>
 </launchSecurity>

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>