conf: fix virDomainDefParseXML for parallels

conf: fix virDomainDefParseXML for parallels

Handle input devices in virDomainDefParseXML properly
in case of parallels containers and VMs.

Parallels containers support only
VIR_DOMAIN_INPUT_BUS_PARALLELS. And if VNC is enabled
we should add implicit mouse and keyboard.

For VMs we should add implicit PS/2 mouse and
keyboard.

BTW, is it worth to refactor code and move
all this code to drivers, to *DomainDefPostParse
functions?

Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>