Medium: Xen: Use xl list $domain return code in status check (boo#901453)

Under xl, state and status are not returned with a --long listing. Instead,
the return code from `xl list $domain` must be used to determine if a domain
is running on the node or not.

As xen-list and xenstore-ls both rely on xenstore, the additiona xenstore-ls
check can safely be removed.

For POSIX compliance, the bash-specific [[ was replaced with expr.
Medium: VirtualDomain: For Xen, prefer xl to xen-list (boo#901453)

When checking pid_status for domains running under Xen, use xl first. If not
available, use xen-list.

An 'emulator' field is not required for PV guests, so change the missing
emulator error message to a debug message.
Dev: Xen: Use tabs for indentation
Merge pull request #513 from krig/xen-agent-updates

medium: Xen: VirtualDomain: Fix issues with using the xl toolset for Xen (boo#901453)