nodedev: Introduce new mdev_types and mdev nodedev capabilities

nodedev: Introduce new mdev_types and mdev nodedev capabilities

The reason for introducing two capabilities, one for the device itself
(cap 'mdev') and one for the parent device listing the available types
('mdev_types'), is that we should be able to do
'virsh nodedev-list --cap' not only for existing mdev devices but also
for devices that support creation of mdev devices, since one day libvirt
might be actually able to create the mdev devices in an automated way
(just like we do for NPIV/vHBA).

https://bugzilla.redhat.com/show_bug.cgi?id=1452072

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