qemu: Add command line for TDX Quote Generation Service(QGS)

qemu: Add command line for TDX Quote Generation Service(QGS)

'tdx-guest' object supports a "quote-generation-socket" property for
attestation purpose. When "quote-generation-socket" is configured in
guest xml, libvirt generates unix socket format cmdline for QEMU.

'Path' element can be omitted, default path "/var/run/tdx-qgs/qgs.socket"
is used in this case.

QEMU command line example:
  qemu-system-x86_64 \
    -object '{"qom-type":"tdx-guest","id":"lsec0","mrconfigid":"xxx","mrowner":"xxx","mrownerconfig":"xxx","quote-generation-socket":{"type":"unix","path":"/var/run/tdx-qgs/qgs.socket"},"attributes":268435457}' \
    -machine pc-q35-6.0,confidential-guest-support=lsec0

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