x86: Optionally avoid injecting AO MCEs while others are pending

x86: Optionally avoid injecting AO MCEs while others are pending

Allow to tell cpu_x86_inject_mce that it should ignore Action Optional
MCE events when the target VCPU is still processing another one. This
will be used by KVM soon.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
CC: Huang Ying <ying.huang@intel.com>
CC: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
CC: Jin Dongming <jin.dongming@np.css.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>