x86: cap address bits CPUID output

x86: cap address bits CPUID output

Don't use more or report more to guests than we are capable of
handling.

At once
- correct the involved extended CPUID level checks,
- simplify the code in hvm_cpuid() and mtrr_top_of_ram().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>