all: Replace virGetLastError with virGetLastErrorCode where we can

Replace instances where we previously called virGetLastError just to
either get the code or to check if an error exists with
virGetLastErrorCode to avoid a validity pre-check.

Signed-off-by: Ramy Elkest <ramyelkest@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

tests/virhostcputest.c

@@ -49,7 +49,7 @@ linuxTestCompareFiles(const char *cpuinfofile,
                                        &nodeinfo.nodes, &nodeinfo.sockets,
                                        &nodeinfo.cores, &nodeinfo.threads) < 0) {
         if (virTestGetDebug()) {
-            if (virGetLastError())
+            if (virGetLastErrorCode())
                 VIR_TEST_DEBUG("\n%s\n", virGetLastErrorMessage());
         }
         VIR_FORCE_FCLOSE(cpuinfo);