configure: qemu-ga: move MSI installer probe after qga probe

configure: qemu-ga: move MSI installer probe after qga probe

MSI probe assumes that qemu-ga support has been probed already, but in
cases where --enable-guest-agent/--disable-guest-agent have not been
passed to configure, qemu-ga support may end up getting enabled later,
as is the case with w32 builds. This leads to MSI probe prematurely
reporting error due to lack of qemu-ga support.

Fix this by moving MSI installer probe after the final qga probes.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>