target/ppc/spapr_caps: Add macro to generate spapr_caps migration vmstate

target/ppc/spapr_caps: Add macro to generate spapr_caps migration vmstate

The vmstate description and the contained needed function for migration
of spapr_caps is the same for each cap, with the name of the cap
substituted. As such introduce a macro to allow for easier generation of
these.

Convert the three existing spapr_caps (htm, vsx, and dfp) to use this
macro.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 1f63ebaa91f73f469c8f107dbd266cabdbea3a40)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>