tpm: Added support for TPM emulator

tpm: Added support for TPM emulator

This change introduces a new TPM backend driver that can communicate with
swtpm(software TPM emulator) using unix domain socket interface. QEMU talks to
the TPM emulator using QEMU's socket-based chardev backend device.

Swtpm uses two Unix sockets for communications, one for plain TPM commands and
responses, and one for out-of-band control messages. QEMU passes the data
socket to be used over the control channel.

The swtpm and associated tools can be found here:
    https://github.com/stefanberger/swtpm

The swtpm's control channel protocol specification can be found here:
    https://github.com/stefanberger/swtpm/wiki/Control-Channel-Specification

Usage:
    # setup TPM state directory
    mkdir /tmp/mytpm
    chown -R tss:root /tmp/mytpm
    /usr/bin/swtpm_setup --tpm-state /tmp/mytpm --createek

    # Ask qemu to use TPM emulator with given tpm state directory
    qemu-system-x86_64 \
        [...] \
        -chardev socket,id=chrtpm,path=/tmp/swtpm-sock \
        -tpmdev emulator,id=tpm0,chardev=chrtpm \
        -device tpm-tis,tpmdev=tpm0 \
        [...]

Signed-off-by: Amarnath Valluri <amarnath.valluri@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>