Makefile: create ".../var/run" when installing the POSIX guest agent

Makefile: create ".../var/run" when installing the POSIX guest agent

Otherwise the default local state directory of POSIX qga won't exist after
installation with a non-standard ${prefix} or DESTDIR.

For now qga is the only user of ".../var" (= $qemu_localstatedir) too, so
don't create that directory either unless we're installing the agent.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>