node_device_conf: Avoid memleak in virNodeDeviceGetPCIVPDDynamicCap()

node_device_conf: Avoid memleak in virNodeDeviceGetPCIVPDDynamicCap()

The virNodeDeviceGetPCIVPDDynamicCap() function is called from
virNodeDeviceGetPCIDynamicCaps() and therefore has to be a wee
bit more clever about adding VPD capability. Namely, it has to
remove the old one before adding a new one. This is how other
functions called from virNodeDeviceGetPCIDynamicCaps() behave
as well.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2143235
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>