backends: Initial support for SPDM socket support

backends: Initial support for SPDM socket support

SPDM enables authentication, attestation and key exchange to assist in
providing infrastructure security enablement. It's a standard published
by the DMTF [1].

SPDM supports multiple transports, including PCIe DOE and MCTP.
This patch adds support to QEMU to connect to an external SPDM
instance.

SPDM support can be added to any QEMU device by exposing a
TCP socket to a SPDM server. The server can then implement the SPDM
decoding/encoding support, generally using libspdm [2].

This is similar to how the current TPM implementation works and means
that the heavy lifting of setting up certificate chains, capabilities,
measurements and complex crypto can be done outside QEMU by a well
supported and tested library.

1: https://www.dmtf.org/standards/SPDM
2: https://github.com/DMTF/libspdm

Signed-off-by: Huai-Cheng Kuo <hchkuo@avery-design.com.tw>
Signed-off-by: Chris Browy <cbrowy@avery-design.com>
Co-developed-by: Jonathan Cameron <Jonathan.cameron@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
[ Changes by WM
 - Bug fixes from testing
]
Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
[ Changes by AF:
 - Convert to be more QEMU-ified
 - Move to backends as it isn't PCIe specific
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20240703092027.644758-3-alistair.francis@wdc.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>