xc: fix segfault in pv domain create if kernel is an invalid image

If libelf calls elf_err() or elf_msg() before elf_set_log() has been
called then it could potentially read an uninitialised log handling
callback function pointer from struct elf_binary. Fix this in libxc by
zeroing the structure before calling elf_init().

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

     1.1 --- a/tools/libxc/xc_hvm_build.c	Mon Aug 09 17:20:39 2010 +0100
     1.2 +++ b/tools/libxc/xc_hvm_build.c	Mon Aug 09 17:43:18 2010 +0100
     1.3 @@ -142,6 +142,7 @@ static int setup_guest(xc_interface *xch
     1.4      if ( memsize > target )
     1.5          pod_mode = 1;
     1.6  
     1.7 +    memset(&elf, 0, sizeof(elf));
     1.8      if ( elf_init(&elf, image, image_size) != 0 )
     1.9          goto error_out;
    1.10      elf_parse_binary(&elf);