roms: assert if max rom size is less than the used size

roms: assert if max rom size is less than the used size

It would ensure that we would notice attempt to write beyond
the allocated buffer. In case of MemoryRegion backed ROM it's
the host buffer and the guest RAM otherwise.

assert can be triggered with:
  dd if=/dev/zero of=/tmp/blob bs=63k count=1
  qemu-system-x86_64 `for  i in {1..33}; do echo -n " -acpitable /tmp/blob"; done`

Fixes: (a1666142db acpi-build: make ROMs RAM blocks resizeable)
Reported-by: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1554982098-336210-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>