hw/moxie/moxiesim: Add support for loading a BIOS on moxiesim

The moxiesim machine already defines a memory region for a firmware,
but does not provide the possibility to load an image via "-bios" yet.
This will be needed for the boot-serial tester, so let's add support
for "-bios" here now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1512031988-32490-6-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>