target-i386: Simplify cpu_x86_find_by_name()

Catch NULL name argument early to avoid repeated checks.
Similarly, check for -cpu host early and untangle from iterating through
model definitions.

Signed-off-by: Andreas Färber <afaerber@suse.de>
target-i386: Drop redundant list of CPU definitions

It is no longer needed since dropping cpudef config file support.
Cleaning this up removes knowledge about other models from x86_def_t,
in preparation for reusing x86_def_t as intermediate step towards pure
QOM X86CPU subclasses.

Signed-off-by: Andreas Färber <afaerber@suse.de>