chown kernel/initrd before spawning qemu

chown kernel/initrd before spawning qemu

If we're running qemu unprivileged, we need to chown any supplied kernel
or initrd before spawning it.

* src/qemu_driver.c: rename qemuDomainSetDiskOwnership() to
  qemuDomainSetFileOwnership(), pass it a path string instead of a disk
  definition and use it for chowning the kernel/initrd in
  qemuDomainSetAllDeviceOwnership()