qemu: Allow forcing read-only mode in qemuDomainStorageSourceAccessModify
qemu: Allow forcing read-only mode in qemuDomainStorageSourceAccessModify

Add a new flag which will set the image as read-only even if the image
data allows writing.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

crobinso/libvirt,andreabolognani/libvirt,libvirt/libvirt,eskultety/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,olafhering/libvirt,eskultety/libvirt,zippy2/libvirt,zippy2/libvirt,jardasgit/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,nertpinx/libvirt,libvirt/libvirt,jardasgit/libvirt,libvirt/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,eskultety/libvirt,eskultety/libvirt,crobinso/libvirt,libvirt/libvirt