x86/MCE: sanitize domain/vcpu ID handling

x86/MCE: sanitize domain/vcpu ID handling

Storing -1 into both fields was misleading consumers: We really should
have a manifest constant for "invalid vCPU" here, and the already
existing DOMID_INVALID should be used.

Also correct a bogus (dead code) check in mca_init_global(), at once
introducing a manifest constant for the early boot "invalid vCPU"
pointer (avoiding proliferation of the open coding). Make that pointer
a non-canonical address at once.

Finally, don't leave mc_domid uninitialized in mca_init_bank().

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