x86: Run qemu_inject_x86_mce on target VCPU

We will use the current TCG-only MCE injection path for KVM as well, and
then this read-modify-write of the target VCPU state has to be performed
synchronously in the corresponding thread.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
