hw/xen: Fix memory leak in libxenstore_open() for Xen

hw/xen: Fix memory leak in libxenstore_open() for Xen

There was a superfluous allocation of the XS handle, leading to it
being leaked on both the error path and the success path (where it gets
allocated again).

Spotted by Coverity (CID 1508098).

Fixes: ba2a92db1ff6 ("hw/xen: Add xenstore operations to allow redirection to internal emulation")
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20230412185102.441523-3-dwmw2@infradead.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>