tpm_emulator: Read control channel response in 2 passes

tpm_emulator: Read control channel response in 2 passes

Error responses from swtpm are typically only 4 bytes long with the
exception of a few commands that return more bytes. Therefore, read the
entire response in 2 steps and stop if the first few bytes indicate an
error response with no subsequent bytes readable. Read the rest in a 2nd
step, if needed. This avoids getting stuck while waiting for too many
bytes in case of an error. The 'getting stuck' condition has not been
observed in practice so far, though.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2615
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>