Add access control support to qemu bridge helper

We go to great lengths to restrict ourselves to just cap_net_admin as an OS
enforced security mechanism.  However, we further restrict what we allow users
to do to simply adding a tap device to a bridge interface by virtue of the fact
that this is the only functionality we expose.

This is not good enough though.  An administrator is likely to want to restrict
the bridges that an unprivileged user can access, in particular, to restrict
an unprivileged user from putting a guest on what should be isolated networks.

This patch implements an ACL mechanism that is enforced by qemu-bridge-helper.
The ACLs are fairly simple whitelist/blacklist mechanisms with a wildcard of
'all'.  All users are blacklisted by default, and deny takes precedence over
allow.

An interesting feature of this ACL mechanism is that you can include external
ACL files.  The main reason to support this is so that you can set different
file system permissions on those external ACL files.  This allows an
administrator to implement rather sophisticated ACL policies based on
user/group policies via the file system.

As an example:

/etc/qemu/bridge.conf root:qemu 0640

 allow br0
 include /etc/qemu/alice.conf
 include /etc/qemu/bob.conf
 include /etc/qemu/charlie.conf

/etc/qemu/alice.conf root:alice 0640
 allow br1

/etc/qemu/bob.conf root:bob 0640
 allow br2

/etc/qemu/charlie.conf root:charlie 0640
 deny all

This ACL pattern allows any user in the qemu group to get a tap device
connected to br0 (which is bridged to the physical network).

Users in the alice group can additionally get a tap device connected to br1.
This allows br1 to act as a private bridge for the alice group.

Users in the bob group can additionally get a tap device connected to br2.
This allows br2 to act as a private bridge for the bob group.

Users in the charlie group cannot get a tap device connected to any bridge.

Under no circumstance can the bob group get access to br1 or can the alice
group get access to br2.  And under no cicumstance can the charlie group
get access to any bridge.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Richa Marwaha <rmarwah@linux.vnet.ibm.com>
Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
