hw: Do not include "sysemu/block-backend.h" if it is not necessary

hw: Do not include "sysemu/block-backend.h" if it is not necessary

After reviewing a patch from Philippe that removes block-backend.h
from hw/lm32/milkymist.c, I noticed that this header is included
unnecessarily in a lot of other files, too. Remove those unneeded
includes to speed up the compilation process a little bit.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1518684912-31637-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>