util: virbuffer: Remove @dynamic from virBufferGetIndent

util: virbuffer: Remove @dynamic from virBufferGetIndent

After the conversion of all callers that would pass true as @dynamic to
a different function we can remove the unused argument now.

Additionally modify the return type to 'size_t' as indentation can't be
negative and remove checks whether @buf is passed as it's caller's duty
to do so.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>