tools/dm_restrict: Ask QEMU to chroot

tools/dm_restrict: Ask QEMU to chroot

When dm_restrict is enabled, ask QEMU to chroot into an empty directory.

* Create $XEN_RUN_DIR/qemu-root-<domid> (deleting the old one if it's there)
* Pass the -chroot option to QEMU

Rather than running `rm -rf` on the directory before creating it
(since there is no library function to do this), simply rmdir the
directory, relying on the fact that the previous QEMU instance, if
properly restricted, shouldn't have been able to write anything
anyway.

Suggested-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v4:
- Minor change to comment
- Update stale directory name in commit message

Changes since v2:
- Style fixes
- Testing moved to a different patch

CC: Ian Jackson <ian.jackson@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Anthony Perard <anthony.perard@citrix.com>