virTypedParamsGetStringList: Ensure that returned array is NULL if there are no match...

virTypedParamsGetStringList: Ensure that returned array is NULL if there are no matching fields

'virTypedParamsGetStringList' fills the returned array only with string
parameters with matching name. The filtering code though leaves the
possibility that all items are filtered out but the return array is
still (over)allocated.

Since 'virTypedParamsFilter()' now also allows filtering by type we can
move the filtering there ensuring that we always allocate the right
number of elements and more importantly the returned array will be NULL
if none elements are present.

Rework the code and adjust docs.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>