Fix compilation with systemtap 1.3

Fix compilation with systemtap 1.3

Version 1.3 of <sys/sdt.h> uses this macro

  #define STAP_CAST(t) (size_t)t

that breaks like this if t is a function

  remote.c:1775: error: cast from function call of type 'const char *'
  to non-matching type 'long unsigned int' [-Wbad-function-cast]

For that to work it should probably look like this

  #define STAP_CAST(t) ((size_t)(t))

In systemtap 1.4 this was completely rewritten.

Anyway, before commit df0b57a95a767c t was always a variable, but now
also a function is used here, namely virNetSASLSessionGetIdentity.

Use an intermediate variable to avoid this problem.