stubs: add qemu_ram_block_from_host() and qemu_ram_get_fd()

stubs: add qemu_ram_block_from_host() and qemu_ram_get_fd()

The blkio block driver will need to look up the file descriptor for a
given pointer. This is possible in softmmu builds where the RAMBlock API
is available for querying guest RAM.

Add stubs so tools like qemu-img that link the block layer still build
successfully. In this case there is no guest RAM but that is fine.
Bounce buffers and their file descriptors will be allocated with
libblkio's blkio_alloc_mem_region() so we won't rely on QEMU's
qemu_ram_get_fd() in that case.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20221013185908.1297568-12-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>