qdev-monitor.c is the only "core qdev" file that is not used in
user-mode emulation, and it does not define anything that is used
by hardware models. Remove it from the hw/ directory and
remove hw/qdev-monitor.h from hw/qdev.h too; this requires
some files to have some new explicitly includes.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>