libxl: Fix possible invalid read

libxl: Fix possible invalid read

According to the following valgrind output, there seems to be a
invalid limit for the iterator (captured on Fedora 19):

==3945== Invalid read of size 1
==3945==    at 0x1E1FA410: libxlVmStart (libxl_driver.c:475)
==3945==    by 0x1E1FAD9A: libxlDomainCreateWithFlags (libxl_driver.c:2633)
==3945==    by 0x5187D46: virDomainCreate (libvirt.c:9439)
==3945==    by 0x13BAA6: remoteDispatchDomainCreateHelper (remote_dispatch.h:2910)
==3945==    by 0x51DE5B9: virNetServerProgramDispatch (virnetserverprogram.c:435)
==3945==    by 0x51D93E7: virNetServerHandleJob (virnetserver.c:165)
==3945==    by 0x50F5BF4: virThreadPoolWorker (virthreadpool.c:144)
==3945==    by 0x50F5670: virThreadHelper (virthreadpthread.c:161)
==3945==    by 0x8046C52: start_thread (pthread_create.c:308)
==3945==    by 0x8758E1C: clone (clone.S:113)
==3945==  Address 0x23424d81 is 0 bytes after a block of size 1 alloc'd
==3945==    at 0x4A08121: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==3945==    by 0x50B1F8C: virAllocN (viralloc.c:189)
==3945==    by 0x1E1FA3CA: libxlVmStart (libxl_driver.c:468)
==3945==    by 0x1E1FAD9A: libxlDomainCreateWithFlags (libxl_driver.c:2633)
==3945==    by 0x5187D46: virDomainCreate (libvirt.c:9439)
==3945==    by 0x13BAA6: remoteDispatchDomainCreateHelper (remote_dispatch.h:2910)
==3945==    by 0x51DE5B9: virNetServerProgramDispatch (virnetserverprogram.c:435)
==3945==    by 0x51D93E7: virNetServerHandleJob (virnetserver.c:165)
==3945==    by 0x50F5BF4: virThreadPoolWorker (virthreadpool.c:144)
==3945==    by 0x50F5670: virThreadHelper (virthreadpthread.c:161)
==3945==    by 0x8046C52: start_thread (pthread_create.c:308)
==3945==    by 0x8758E1C: clone (clone.S:113)
==3945==

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1013045
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>