build: skip qemu in tests when !WITH_QEMU

A mingw build (where the qemu driver is not built, so WITH_QEMU
is undefined) failed with:

In file included from ../../src/qemu/qemu_command.h:30:0,
                 from ../../tests/testutilsqemu.h:4,
                 from ../../tests/networkxml2xmltest.c:14:
../../src/qemu/qemu_conf.h:53:4: error: #error "Port me"

But since testutilsqemu.c is already conditional, the header
should be likewise.

* tests/testutilsqemu.h: Make content conditional.
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/testutilsqemu.h

+#ifdef WITH_QEMU
 
-#include "capabilities.h"
-#include "domain_conf.h"
-#include "qemu/qemu_command.h"
+# include "capabilities.h"
+# include "domain_conf.h"
+# include "qemu/qemu_command.h"
 
 virCapsPtr testQemuCapsInit(void);
 virDomainXMLOptionPtr testQemuXMLConfInit(void);
 extern qemuBuildCommandLineCallbacks testCallbacks;
+#endif