On RHEL 5, with dbus 1.1.2, compilation failed with:

virsystemdmock.c: In function 'dbus_connection_send_with_reply_and_block':
virsystemdmock.c:68: warning: implicit declaration of function 'dbus_message_set_serial'

Fix this by instead bypassing all attempts to use a dbus serial.

* tests/virsystemdmock.c (dbus_message_set_reply_serial): Add new
override.
(dbus_connection_send_with_reply_and_block): No longer bother with
the serial.
Signed-off-by: NEric Blake <eblake@redhat.com>

A cross-compile to mingw failed:

  CC       virsystemdmock_la-virsystemdmock.lo
../../tests/virsystemdmock.c:29:6: error: 'dbus_connection_set_change_sigpipe' redeclared without dllimport attribute: previous dllimport ignored [-Werror=attributes]
 void dbus_connection_set_change_sigpipe(dbus_bool_t will_modify_sigpipe ATTRIBUTE_UNUSED)
      ^

But when you think about it, systemd is Linux-only, and even our
use of LD_PRELOAD to provide mock syscalls is Linux-only.

* tests/virsystemdmock.c: Avoid compilation outside Linux.
Signed-off-by: NEric Blake <eblake@redhat.com>

If systemd machine does not exist, return -2 instead of -1,
so that applications don't need to repeat the tedious error
checking code
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

To register virtual machines and containers with systemd-machined,
and thus have cgroups auto-created, we need to talk over DBus.
This is somewhat tedious code, so introduce a dedicated function
to isolate the DBus call in one place.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>