hw/xen/xen_pt: fix uninitialized variable

hw/xen/xen_pt: fix uninitialized variable

xen_pt_config_reg_init() reads only that many bytes as the size of the
register that is being initialized. It uses
xen_host_pci_get_{byte,word,long} and casts its last argument to
expected pointer type. This means for smaller registers higher bits of
'val' are not initialized. Then, the function fails if any of those
higher bits are set.

Fix this by initializing 'val' with zero.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Message-Id: <20230127050815.4155276-1-marmarek@invisiblethingslab.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>