qemu-common.h should only be included by .c files.  Its file comment
explains why: "No header file should depend on qemu-common.h, as this
would easily lead to circular header dependencies."

hw/hw.h includes qemu-common.h, but its users generally need only
hw_error() and qemu/module.h from it.  Move the former to hw/hw.h,
include the latter there, and drop the ill-advised include.
hw/misc/cbus.c now misses hw_error(), so include hw/hw.h there.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>