MAINTAINERS: Add myself as Guest Agent co-maintainer

Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Acked-by: Michael Roth <michael.roth@amd.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index ead2bed..6af9cd9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2881,7 +2881,7 @@
 
 QEMU Guest Agent
 M: Michael Roth <michael.roth@amd.com>
-R: Konstantin Kostiuk <kkostiuk@redhat.com>
+M: Konstantin Kostiuk <kkostiuk@redhat.com>
 S: Maintained
 F: qga/
 F: docs/interop/qemu-ga.rst