pl022: fix buffer overun on invalid state load

CVE-2013-4530

pl022.c did not bounds check tx_fifo_head and
rx_fifo_head after loading them from file and
before they are used to dereference array.

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