build: fix type-punning bug

build: fix type-punning bug

With older gcc and 64-bit size_t, the compiler issues a real warning:
rpc/virnetserverservice.c:277: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

Introduced in commit 0cc79255.  Depending on machine endianness,
this warning represents a real bug that could mis-interpret the
value by a factor of 2^32.  I don't know why I couldn't get newer
gcc to report the same warning message.

* src/rpc/virnetserverservice.c
(virNetServerServiceNewPostExecRestart): Use temporary instead.