To allow their use from other source files, move qemuDriverLock
and qemuDriverUnlock to qemu_conf.h and make them non-static

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Add qemuDriverLock
  qemuDriverUnlock
* src/qemu/qemu_driver.c: Remove qemuDriverLock and qemuDriverUnlock