control channel: add error logs upon receiving short input

control channel: add error logs upon receiving short input

There was a recent failure when migrating a production VM
> kvm: tpm-emulator: Setting the stateblob (type 1) failed with a TPM error 0x3 a parameter is bad
> kvm: error while loading state for instance 0x0 of device 'tpm-emulator'
> kvm: load of migration failed: Input/output error
and it's not clear what exactly triggered it. Note that 0x3 is
TPM_BAD_PARAMETER.

The timeout check when receiving the command+body is a good candidate,
but since poll() is called with the remaining timeout, it seems hard
to trigger. In both cases with short input, QEMU would've not managed
to even send the full header (16 bytes for CMD_SET_STATEBLOB).

Another possibility is that the blob header got corrupted and the
TPM_BAD_PARAMETER error orignated from the SWTPM_NVRAM_SetStateBlob()
call. The checks there already have logging.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>