kvm tools: Teach 'run' to handle guestfs

This patch allows to run previously created guestfs by simply specifying it
with the '-d' parameter.

This allows running guestfs which were created before using:

	kvm setup -n [name]

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>