Current Makefile system allows using foo.o-cflags variables to store
object-specific CFLAGS.  Convert some usages of old syntax
(using QEMU_CFLAGS += construct) to the new syntax.

Do not touch multifile modules for now, as build system isn't ready for this.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>