Use do_machine_check instead of INT $12 to pass MCE to the host,
the same approach VMX uses.

On a related note, there is no reason to limit the use of do_machine_check
to 64 bit targets, as is currently done for VMX. MCE handling works
for both target families.

The patch is only compile tested, for both, 64 and 32 bit targets,
someone should test the passing of the exception by injecting
some MCEs into the guest.

For future non-RFC patch, kvm_machine_check should be moved to some
appropriate header file.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Message-Id: <20200411153627.3474710-1-ubizjak@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>