rpc: virnetserverclient: Introduce new attribute conn_time to client

rpc: virnetserverclient: Introduce new attribute conn_time to client

Besides ID, libvirt should provide several parameters to help the user
distinguish two clients from each other. One of them is the connection
timestamp. This patch also adds a testcase for proper JSON formatting of the
new attribute too (proper formatting of older clients that did not support
this attribute yet is included in the existing tests) - in order to
testGenerateJSON to work, a mock of time_t time(time_t *timer) needed to be
created.

Signed-off-by: Erik Skultety <eskultet@redhat.com>