Description of problem:
[Text] - Improve the error message when failing to run VM with network-used host device.

When failing to run VM with network-used host device, the next error message is displayed:

"Error while executing action: 

sr-vm3:
Cannot run VM. There is no host that satisfies current scheduling constraints. See below for details:
The host puma22.scl.lab.tlv.redhat.com did not satisfy internal filter HostDevice because some of the required host devices are unavailable.."

This error message should be improved, because it is really not clear why it's unavailable.
The error message should explain that the host devices are unavailable because they are in use by some network/s or something like that.

Version-Release number of selected component (if applicable):
3.6.3-0.1

How reproducible:
100

Steps to Reproduce:
1. Add device to VM via Host Devices sub tab that has a network attached to him
2. Try to run VM

Actual results:
Failed with error:
"Error while executing action: 

sr-vm3:
Cannot run VM. There is no host that satisfies current scheduling constraints. See below for details:
The host puma22.scl.lab.tlv.redhat.com did not satisfy internal filter HostDevice because some of the required host devices are unavailable.."

Expected results:
Error message should be improved.

Additional info:
See also - BZ 1230775