gnu: qemu: Update to 2.9.0 [security fixes].

Fixes CVE-2017-{5857,5973,5987,6058,6505,7377,7471,7718}.

* gnu/packages/qemu.scm (qemu): Update to 2.9.0.
[source]: Remove obsolete patches.
* gnu/packages/patches/qemu-CVE-2016-10155.patch,
gnu/packages/patches/qemu-CVE-2017-5525.patch,
gnu/packages/patches/qemu-CVE-2017-5526.patch,
gnu/packages/patches/qemu-CVE-2017-5552.patch,
gnu/packages/patches/qemu-CVE-2017-5578.patch,
gnu/packages/patches/qemu-CVE-2017-5579.patch,
gnu/packages/patches/qemu-CVE-2017-5856.patch,
gnu/packages/patches/qemu-CVE-2017-5898.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Remove them.