libxc: don't pass uninitialized data to do_dm_op()

libxc: don't pass uninitialized data to do_dm_op()

do_dm_op() expects (void *, size_t) pairs, but with nr being uint32_t
the type of the expression of xc_hvm_track_dirty_vram()'s last argument
to the function is only a 32 bits one. Neither C nor the ABI require
the compiler to promote the type beyond int.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>