libxl: libxl__xs_transaction_commit should always clear the transaction.

libxl: libxl__xs_transaction_commit should always clear the transaction.

This includes the EAGAIN case.

Users are of the form:

   xs_transaction_t t = 0;

   for (;;) {
        rc = libxl__xs_transaction_start(gc, &t);

	rc = stuff
	if (rc) goto out;
	...more...

        rc = libxl__xs_transaction_commit(gc, &t);
        if (!rc) break;
        if (rc<0) goto out;
    }
  ...
 out:

So in EAGAIN (commit -> +1) we will go round the loop again and call start
which leads to:
    xl: libxl_xshelp.c:174: libxl__xs_transaction_start: Assertion `!*t' failed.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>