virtio: fix virtio-blk under debug allocator

The debug allocator can allocate non-contiguous memory for large requests,
but since b7de9871 it uses only one sg entry for the entire buffer.

One possible fix is to allocate contiguous memory even under the debug
allocator, but in the future we may wish to allow discontiguous allocation
when not enough contiguous space is available.  So instead we implement
a virt_to_phys() variant that takes a range, and outputs the physical
segments that make it up, and use that to construct a minimal sg list
depending on the input.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>