passthrough: fix buffer overflow of vslots

passthrough: fix buffer overflow of vslots

Assuming we assign n devices, strlen(direct_pci) can be 13n and the
length of the old 'vslots' is 13n/3 which is smaller than 5n+1 (1
slot_str takes 5 bytes).  So we have to malloc a bigger buffer for
vslots.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>