hostmem: fix QEMU crash by 'info memdev'

'info memdev' crashes QEMU:
   (qemu) info memdev
   Unexpected error in parse_str() at qapi/string-input-visitor.c:111:
   Parameter 'null' expects an int64 value or range
It is caused by null uint16List is returned if 'host-nodes' is the default
value

Return MAX_NODES under this case to fix this bug

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
