xenopsd: Support driver domains for PV guests

The case of HVM guests with emulated QEMU networks devices is not yet
handled.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Allow setting the VIF backend for a network

If, on a network, other_config:backend_vm is set to the UUID of a VM, this VM is
used for the VIF backends (netback) of any VIFs on the network.

This is useful for internal networks (only), since the key is set on a network
rather than on a PIF.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
CP-2190 for PR-1221: SR alerts for hosts can be configured on the SR now.

To reduce the effort of configuration, whereas previously it was
necessary to configure throughput alerts for an SR on each host
individually, now it is possible to set up a per-host SR throughput
alert for each host connected to an SR by writing to the SR's
other-config:perfmon key.
CA-101788: increase xenconsoled default number of fds in xenservices from 1024 to 4096

xenconsoled is currently started in xenservices using the default ulimit -n
value 1024. This aligns well with the limit of 1024 fds that the xenconsoled
select() can handle. Each vm console will tipically use 3 fds (4 if logging
is enabled), limiting the number of consoles in dom0 to a couple of hundred.

The new upstream xenconsoled uses poll() instead of select() and can handle
>1024 fds, as long as its ulimit -n value is properly set up in xenservices.
Increasing this value to 4096 will allow us to have 1000 vm consoles in dom0,
which is more aligned to other values.

Signed-off-by: Marcus Granado <marcus.granado@citrix.com>
Merge pull request #1111 from mg12/CA-101788

CA-101788: increase xenconsoled default number of fds in xenservices fro...
Merge pull request #1036 from jeromemaloberti/CP-2190-master

CP-2190 for PR-1221: SR alerts for hosts can be configured on the SR now.
Merge pull request #1002 from robhoes/p2p-master

Allow setting the VIF backend for a network