qemu: Add callback struct for qemuBuildCommandLine

Since 0d70656afd, it starts to access the sysfs files to build
the qemu command line (by virSCSIDeviceGetSgName, which is to find
out the scsi generic device name by adpater🚌target:unit), there
is no way to work around, qemu wants to see the scsi generic device
like "/dev/sg6" anyway.

And there might be other places which need to access sysfs files
when building qemu command line in future.

Instead of increasing the arguments of qemuBuildCommandLine, this
introduces a new callback for qemuBuildCommandLine, and thus tests
can register their own callbacks for sysfs test input files accessing.

* src/qemu/qemu_command.h: (New callback struct
                            qemuBuildCommandLineCallbacks;
                            extern buildCommandLineCallbacks)
* src/qemu/qemu_command.c: (wire up the callback struct)
* src/qemu/qemu_driver.c: (Use the new syntax of qemuBuildCommandLine)
* src/qemu/qemu_hotplug.c: Likewise
* src/qemu/qemu_process.c: Likewise
* tests/testutilsqemu.[ch]: (Helper testSCSIDeviceGetSgName;
                             callback struct testCallbacks;)
* tests/qemuxml2argvtest.c: (Use testCallbacks)
* src/tests/qemuxmlnstest.c: (Like above)

tests/testutilsqemu.h

@@ -1,6 +1,8 @@
 
 #include "capabilities.h"
 #include "domain_conf.h"
+#include "qemu/qemu_command.h"
 
 virCapsPtr testQemuCapsInit(void);
 virDomainXMLOptionPtr testQemuXMLConfInit(void);
+extern qemuBuildCommandLineCallbacks testCallbacks;