uuid: fix possible non-terminated string

uuid: fix possible non-terminated string

Error: STRING_NULL:
/libvirt/src/util/uuid.c:273:
string_null_argument: Function "getDMISystemUUID" does not terminate string "*dmiuuid".
/libvirt/src/util/uuid.c:241:
string_null_argument: Function "saferead" fills array "*uuid" with a non-terminated string.
/libvirt/src/util/util.c:101:
string_null_argument: Function "read" fills array "*buf" with a non-terminated string.
/libvirt/src/util/uuid.c:274:
string_null: Passing unterminated string "dmiuuid" to a function expecting a null-terminated string.
/libvirt/src/util/uuid.c:138:
var_assign_parm: Assigning: "cur" = "uuidstr". They now point to the same thing.
/libvirt/src/util/uuid.c:164:
string_null_sink_loop: Searching for null termination in an unterminated array "cur".