qemu: Validate backing store of 'mirror' for block copy

qemu: Validate backing store of 'mirror' for block copy

Since 4e797f1a we parse backingStore of mirror which will later be used
with blockdev. Add some validation for the user passed mirror at the
current point to make sure it's not used improperly.

Validate that it's not used without blockdev and also that it's not
passed when not requesting a shallow copy. Also add a chain terminator
for a deep copy since we know the resulting mirror will not have chain.

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