When running QEMU with "-cpu ?" we walk through every alias for every
target CPU we know about. This takes several seconds on my very fast
host system.

Let's introduce a class object cache in the alias table. Using that we
don't have to go through the tedious work of finding our target class.
Instead, we can just go directly from the alias name to the target class
pointer.

This patch brings -cpu "?" to reasonable times again.

Before:
  real    0m4.716s

After:
  real    0m0.025s
Signed-off-by: NAlexander Graf <agraf@suse.de>