allow overriding of CPUID level on command line

The CPUID level determines how many CPUID leafs are exposed to the guest.
Some features (like multi-core) cannot be propagated without the proper
level, but guests maybe confused by bogus entries in some leafs.
So add level= and xlevel= to the list of -cpu options to allow the user to
override the default settings. While at it, merge unnecessary local
variables into one and allow hexadecimal arguments.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
