chroot and change user support (Nolan)

chroot and change user support (Nolan)

Resent with fixed formatting.

This patch adds two new command line options:
 -chroot <dir>
 -runas <user>

This is useful for running qemu as an unprivileged user in a chroot
jail.  To avoid having to populate the jail, chrooting happens right
before the start of guest execution.

Signed-off-by: Nolan Leake <nolan@sigbus.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6652 c046a42c-6fe2-441c-8c8c-71466251a162