x86/cpuid: Handle leaf 0x80000000 in guest_cpuid()

x86/cpuid: Handle leaf 0x80000000 in guest_cpuid()

The calculations for p->extd.max_leaf are reworked to force a value of at
least 0x80000000, and to take the domains chosen vendor into account when
clamping maximum value.

The high short vendor information is clobbered or duplicated according to the
chosen vendor.

As a side effect of handing out an audited max_leaf value, the 0x8000001e case
can be dropped from pv_cpuid(), as it outside of the visible range.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Jan Beulich <JBeulich@suse.com>