stubs: Add arch_type

blockdev.c uses the arch_type constant, so before we can use the file in
tools (i.e. outside of the system emulator), we need to add a stub for
it. A new QEMU_ARCH_NONE is introduced for this case.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-3-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>