gnu: qemu: Update to 2.5.0; add fixes for security flaws.

* gnu/packages/patches/qemu-CVE-2015-6855.patch: Delete file.
* gnu/packages/patches/qemu-virtio-9p-use-accessor-to-get-thread-pool.patch,
  gnu/packages/patches/qemu-CVE-2015-8558.patch,
  gnu/packages/patches/qemu-CVE-2015-8567.patch,
  gnu/packages/patches/qemu-CVE-2015-8613.patch,
  gnu/packages/patches/qemu-CVE-2015-8701.patch,
  gnu/packages/patches/qemu-CVE-2015-8743.patch,
  gnu/packages/patches/qemu-CVE-2016-1568.patch,
  gnu/packages/patches/qemu-CVE-2016-1922.patch: New files.
* gnu-system.am (dist_patch_DATA): Remove 'qemu-CVE-2015-6855.patch'; add the
  new patches.
* gnu/packages/qemu.scm (qemu): Update to 2.5.0.
  [source]: Remove old patches and add new ones.
  [arguments]: Add 'disable-test-qga' phase.
  (%glib-memory-vtable-patch, %glib-duplicate-test-patch): Remove variables.