vtpmmgr: Support GetRandom passthrough on TPM 2.0

vtpmmgr: Support GetRandom passthrough on TPM 2.0

GetRandom passthrough currently fails when using vtpmmgr with a hardware
TPM 2.0.
vtpmmgr (8): INFO[VTPM]: Passthrough: TPM_GetRandom
vtpm (12): vtpm_cmd.c:120: Error: TPM_GetRandom() failed with error code (30)

When running on TPM 2.0 hardware, vtpmmgr needs to convert the TPM 1.2
TPM_ORD_GetRandom into a TPM2 TPM_CC_GetRandom command.  Besides the
differing ordinal, the TPM 1.2 uses 32bit sizes for the request and
response (vs. 16bit for TPM2).

Place the random output directly into the tpmcmd->resp and build the
packet around it.  This avoids bouncing through an extra buffer, but the
header has to be written after grabbing the random bytes so we have the
number of bytes to include in the size.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed by: Daniel P. Smith <dpsmith@apertussolutions.com>