MAINTAINERS: Take over 0.15 maintenance

SUSE is shipping qemu-kvm 0.15.1 with SLES 11 SP2 so we will be actively
tracking all KVM-related issues. Therefore upgrade to Supported.

Signed-off-by: Andreas Färber <afaerber@suse.de>

MAINTAINERS

@@ -852,8 +852,9 @@ S: Orphan
 
 Stable 0.15
 L: qemu-stable@nongnu.org
+M: Andreas Färber <afaerber@suse.de>
 T: git git://git.qemu.org/qemu-stable-0.15.git
-S: Orphan
+S: Supported
 
 Stable 0.14
 L: qemu-stable@nongnu.org