util/error: allow non-NUL-terminated err->src

util/error: allow non-NUL-terminated err->src

Rust makes the current file available as a statically-allocated string,
but without a NUL terminator.  Allow this by storing an optional maximum
length in the Error.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>