tools/libxc/xc_dom_arm: add missing variable initialization

The variable domctl.u.address_size.size may remain uninitialized if
guest_type is not one of xen-3.0-aarch64 or xen-3.0-armv7l. And the
code precisely checks if this variable is still 0 to decide if the
guest type is supported or not.

This fixes the following build failure with gcc 7.x:

xc_dom_arm.c:229:31: error: 'domctl.u.address_size.size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     if ( domctl.u.address_size.size == 0 )

Patch originally taken from
https://www.mail-archive.com/xen-devel@lists.xen.org/msg109313.html.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 88bfbf90e3)

tools/libxc/xc_dom_arm.c

@@ -223,6 +223,8 @@ static int set_mode(xc_interface *xch, domid_t domid, char *guest_type)
 
     domctl.domain = domid;
     domctl.cmd    = XEN_DOMCTL_set_address_size;
+    domctl.u.address_size.size = 0;
+
     for ( i = 0; i < ARRAY_SIZE(types); i++ )
         if ( !strcmp(types[i].guest, guest_type) )
             domctl.u.address_size.size = types[i].size;