selftests/bpf: Propagate error code of the command to vmtest.sh

When vmtest.sh ran a command in a VM, it did not record or propagate the
error code of the command. This made the script less "script-able". The
script now saves the error code of the said command in a file in the VM,
copies the file back to the host and (when available) uses this error
code instead of its own.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210225161947.1778590-1-kpsingh@kernel.org