xilinx: fix buffer overflow on realize

ASAN complains about buffer overflow when running:
aarch64-softmmu/qemu-system-aarch64 -machine xilinx-zynq-a9

==476==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000035e38 at pc 0x000000f75253 bp 0x7ffc597e0ec0 sp 0x7ffc597e0eb0
READ of size 8 at 0x602000035e38 thread T0
    #0 0xf75252 in xilinx_spips_realize hw/ssi/xilinx_spips.c:623
    #1 0xb9ef6c in device_set_realized hw/core/qdev.c:918
    #2 0x129ae01 in property_set_bool qom/object.c:1854
    #3 0x1296e70 in object_property_set qom/object.c:1088
    #4 0x129dd1b in object_property_set_qobject qom/qom-qobject.c:27
    #5 0x1297168 in object_property_set_bool qom/object.c:1157
    #6 0xb9aeac in qdev_init_nofail hw/core/qdev.c:358
    #7 0x78a5bf in zynq_init_spi_flashes /home/elmarco/src/qemu/hw/arm/xilinx_zynq.c:125
    #8 0x78af60 in zynq_init /home/elmarco/src/qemu/hw/arm/xilinx_zynq.c:238
    #9 0x998eac in main /home/elmarco/src/qemu/vl.c:4534
    #10 0x7f96ed692730 in __libc_start_main (/lib64/libc.so.6+0x20730)
    #11 0x41d0a8 in _start (/home/elmarco/src/qemu/aarch64-softmmu/qemu-system-aarch64+0x41d0a8)

0x602000035e38 is located 0 bytes to the right of 8-byte region [0x602000035e30,0x602000035e38)
allocated by thread T0 here:
    #0 0x7f970b014e60 in malloc (/lib64/libasan.so.3+0xc6e60)
    #1 0x7f96f15b0e18 in g_malloc (/lib64/libglib-2.0.so.0+0x4ee18)
    #2 0xb9ef6c in device_set_realized hw/core/qdev.c:918
    #3 0x129ae01 in property_set_bool qom/object.c:1854
    #4 0x1296e70 in object_property_set qom/object.c:1088
    #5 0x129dd1b in object_property_set_qobject qom/qom-qobject.c:27
    #6 0x1297168 in object_property_set_bool qom/object.c:1157
    #7 0xb9aeac in qdev_init_nofail hw/core/qdev.c:358
    #8 0x78a5bf in zynq_init_spi_flashes /home/elmarco/src/qemu/hw/arm/xilinx_zynq.c:125
    #9 0x78af60 in zynq_init /home/elmarco/src/qemu/hw/arm/xilinx_zynq.c:238
    #10 0x998eac in main /home/elmarco/src/qemu/vl.c:4534
    #11 0x7f96ed692730 in __libc_start_main (/lib64/libc.so.6+0x20730)

s->spi is allocated with the size of num_busses which may be 1 (by
default).  Change to use a loop up to s->num_busses also for the
call to ssi_auto_connect_slaves().

Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
