libxc, libxl, hvmloader: strip out outdated VM generation ID implementation

libxc, libxl, hvmloader: strip out outdated VM generation ID implementation

The VM generation ID support in libxc/libxl was based on a draft
specification which subsequently changed considerably.  Remove much of
the code in anticipation of introducing something simpler that
conforms to the current specification from Microsoft.

Switch to using a HVM param (HVM_PARAM_VM_GENERATION_ID_ADDR) instead
of the hvmloader/generation-id-address XenStore key.  This simplifies
save/restore since it only needs to transfer the value of this param.

There are no changes to the migration stream format or the public
libxl API.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- updated xc_nomigrate.c for interface change ]