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ExtStorage Interface support for userspace access

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Overview

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The ExtStorage Interface gets extended to cater for ExtStorage providers

that support userspace access. This will allow the instances to access

their external storage devices directly without going through a block

device, avoiding expensive context switches with kernel space and the

potential for deadlocks in low memory scenarios. The implementation

should be backwards compatible and allow existing ExtStorage

providers to work as is.

Implementation

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Since the implementation should be backwards compatible we are not going

to add a new script in the set of scripts an ExtStorage provider should

ship with. Instead, the 'attach' script, which is currently responsible

to map the block device and return a valid device path, should also be

responsible for providing the URIs that will be used by each

hypervisor. Even though Ganeti currently allows userspace access only

for the KVM hypervisor, we want the implementation to enable the

extstorage providers to support more than one hypervisors for future

compliance.

More specifically, the 'attach' script will be allowed to return more

than one line. The first line will contain as always the block device

path. Each one of the extra lines will contain a URI to be used for the

userspace access by a specific hypervisor. Each URI should be prefixed

with the hypervisor it corresponds to (e.g. kvm:<uri>). The prefix will

be case insensitive. If the 'attach' script doesn't return any extra

lines, we assume that the ExtStorage provider doesn't support userspace

access (this way we maintain backward compatibility with the existing

'attach' scripts).

The 'GetUserspaceAccessUri' method of the 'ExtStorageDevice' class will

parse the output of the 'attach' script and if the provider supports

userspace access for the requested hypervisor, it will use the

corresponding URI instead of the block device itself.