vmstate: fix buffer overflow in target-arm/machine.c

CVE-2013-4531

cpreg_vmstate_indexes is a VARRAY_INT32. A negative value for
cpreg_vmstate_array_len will cause a buffer overflow.

VMSTATE_INT32_LE was supposed to protect against this
but doesn't because it doesn't validate that input is
non-negative.

Fix this macro to valide the value appropriately.

The only other user of VMSTATE_INT32_LE doesn't
ever use negative numbers so it doesn't care.

Reported-by: Anthony Liguori <anthony@codemonkey.ws>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
