hw/xen: Add backend implementation of grant table operations

hw/xen: Add backend implementation of grant table operations

This is limited to mapping a single grant at a time, because under Xen the
pages are mapped *contiguously* into qemu's address space, and that's very
hard to do when those pages actually come from anonymous mappings in qemu
in the first place.

Eventually perhaps we can look at using shared mappings of actual objects
for system RAM, and then we can make new mappings of the same backing
store (be it deleted files, shmem, whatever). But for now let's stuck to
a page at a time.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>