qemu: blockcopy: sanitize permission handling for 'mirror'

qemu: blockcopy: sanitize permission handling for 'mirror'

At the point when we want to modify the permissions for the 'mirror' we
know whether it is supposed to have a backing chain or no. Given that
mirror->backingStore is populated only when we'd need to touch it ayways
we can use qemuDomainStorageSourceChainAccessAllow even in place of
qemuDomainStorageSourceAccessAllow used for other cases to simplify the
code.

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