Libvirt: Add missing default value for config option max_queued_clients

Libvirt: Add missing default value for config option max_queued_clients

Commit 1199edb1d4e3 added config option max_queued_clients and documented the
default value as 1000 but never actually set that value. This patch sets the
default value.

This addresses an issue whereby the following error message is reported if too
many migrations are started simultaneously:

error: End of file while reading data: Ncat: Invalid argument.: Input/output error

The problem is that too many ncat processes are spawned on the destination
system. They all attempt to connect to the libvirt socket. Because the
destination libvirtd cannot respond to the connect requests quickly enough we
overrun the socket's pending connections queue.

Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>