[virt-tools-list] Make setKeepAlive a preference

[Charles Arnold]

With large Xen VMs (>128Gig) the libvirt connection will sometimes
timeout while Xen is scrubing memory.  I don't know if KVM would
have similar issues.  Setting the interval longer or increasing the retry
count in the call to _backend.setKeepAlive is a simple way to solve the
problem but perhaps not the best.

What I wonder is if there is interest in creating a preferences tab
allowing these two values to be modified as needed.  Or maybe
it should go on the Connection Details Overview dialog next to the
Autoconnect checkbox?  I could put together a patch if doing this is
desirable.

- Charles