NVMe: Separate controller init from disk discovery

This combines the controller initialization into one function, removing
IO queue setup from namespace discovery, and creates symetric functions
for device removal. The controller start and shutdown functions can now
be called from resume/suspend context as well as probe/remove.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>