MAINTAINERS: Add myself as Guest Agent reviewer

Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-Id: <20220712092715.2136898-1-kkostiuk@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>

MAINTAINERS

@@ -2880,6 +2880,7 @@ T: git https://repo.or.cz/qemu/armbru.git qapi-next
 
 QEMU Guest Agent
 M: Michael Roth <michael.roth@amd.com>
+R: Konstantin Kostiuk <kkostiuk@redhat.com>
 S: Maintained
 F: qga/
 F: docs/interop/qemu-ga.rst