Add Mirage 1.1.0 release

This consists of a series of fairly interlocked packages, so all the
constraints have been updated in one pull request.  With this release, all OPAM
package conflicts have been removed, so it's far easier to explicitly choose at
link time which packages should be linked.

* The `mirage-types` library now fetches the types directly from the
  `mirage/mirage` repository to reduce the number of separate git repositories we
  have to track.

* Several major libraries have also been functorized to eliminate the need to
  compile them separately with Unix and Xen.  Specifically,
  `mirage-tcpip-{unix/xen}` is now deprecated in favor of `tcpip`, as is
  `mirage-http-{unix/xen}` in favor of `mirage-http`.

* The block libraries have been refreshed to use `V1_LWT` interface, and are more
  efficient at random I/O (for VHD) and use larger request sizes (for the Xen
  block driver)

* The `xenctrl` package has been replaced with `xen-evtchn` and `xen-gnt`
  to continue the support for parallel package installation without conflicts.