x86: Refine error reporting of MCE injection services

As this service is used by the human monitor, make sure that errors get
reported to the right channel, and also raise the verbosity.

This requires to move Monitor typedef in qemu-common.h to resolve the
include dependency.

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>
