builder/amazon/common: handle missing source AMI error

Specifying a non-existent source AMI will result in an empty array of
images in the response which would cause an "index out of range" panic.
This change handles this case more gracefully with an error message.

/cc @rasa - This is more idiomatic Go: to share the platform-independent
stuff and make the platform-specific stuff unexported functions

This adds support for Windows VMware Workstation

Conflicts:
	builder/amazon/common/run_config.go

This adds "no-launch" EBS-backed AMI creation, which allows the creation
of these AMIs without launching a new EC2 instance. Instead this builder
is meant to be run on an existing EC2 instance and uses that instance as
a host to attach root EBS volumes, provision in a chroot, etc.

Was getting weird behavior... see
https://groups.google.com/d/msg/golang-nuts/a1kymwSVt2M/FwcCuBl1_48