proxyquery: move __init__ to the top of class

The constructor of ProxyQueryMiddleware was defined below another
method. It's nicer style to have the constructor always at the top.
proxyquery: rename `PQM.zerovm_sysimage_devices`

The `zerovm_sysimage_devices` attribute of `ProxyQueryMiddleware`
has been renamed to `zerovm_sysimage_device_names`.

Tests updated accordingly.
proxyquery: factor out `get_sysimage_device_names`
proxyquery: simplify `get_sysimage_device_names`
proxyquery: deprecate `zerovm_sysimage_devices`

For the ProxyQueryMiddleware, the `zerovm_sysimage_devices` config
param has been deprecated and replaced with
`zerovm_sysimage_device_names`.

The old name is still supported.