dma: make dma access its own address space

Instead of accessing the cpu address space, use an address space
configured by the caller.

Eventually all dma functionality will be folded into AddressSpace,
but we have to start from something.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>