x86/cpuid: Handle leaf 0x80000001 in guest_cpuid()

x86/cpuid: Handle leaf 0x80000001 in guest_cpuid()

Intel reserve eax and ebx, while AMD duplicates eax from the low
family/model/stepping leaf.  For AMD, ebx contains further brand/package
information which is left as the toolstack chooses (other than bits 27:16
which are reserved).

While moving the dynamic adjustments from the legacy path, simplify the shadow
PSE36 adjustment.  PAE paging is a prerequisite for enabling long mode, making
the long mode check redundant; the case where it doesn't get short circuited
is the case where it is architecturally 0.  Make the same adjustment to the
leaf 1 legacy path.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>