qemu: fix resource leak

qemu: fix resource leak

Error: RESOURCE_LEAK:
/libvirt/src/qemu/qemu_driver.c:6968:
alloc_fn: Calling allocation function "calloc".
/libvirt/src/qemu/qemu_driver.c:6968:
var_assign: Assigning: "nodeset" =  storage returned from "calloc(1UL, 1UL)".
/libvirt/src/qemu/qemu_driver.c:6977:
noescape: Variable "nodeset" is not freed or pointed-to in function "virTypedParameterAssign".
/libvirt/src/qemu/qemu_driver.c:6997:
leaked_storage: Variable "nodeset" going out of scope leaks the storage it points to.