allow overriding of CPUID level on command line

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>