x86/cpuid: Handle leaf 0x5 in guest_cpuid()

x86/cpuid: Handle leaf 0x5 in guest_cpuid()

The MONITOR flag isn't exposed to guests.  The existing toolstack logic, and
pv_cpuid() in the hypervisor, zero the MONITOR leaf for queries.

However, the MONITOR leaf is still visible in the hardware domains native
CPUID view, and Linux depends on this to set up C-state information.  Leak the
hosts MONITOR leaf under the same circumstances that the MONITOR feature is
leaked.

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