domain: Require <init> for container guests

domain: Require <init> for container guests

Use capabilities to allow a driver to register a default <init> if none
is specified in the XML. Openvz was already open-coding this to be /sbin/init

LXC currently falls over if no init is specified, so an explicit error is
an improvement IMO.

(Side note: I don't think we can set a default value for LXC. If we use
/sbin/init but the user doesn't specify a separate root FS for their guest,
the container will rerun the host's init which can be traumatic :). For
virt-install I'm thinking of defaulting to /sbin/init if a root FS has
been specified, otherwise require the user to manually specify <init>)