qemuBlockStorageSourceGetBackendProps: Report errors on all switch cases
qemuBlockStorageSourceGetBackendProps: Report errors on all switch cases

Few switch cases returned failure but didn't report an error. For a
situation when the backingStore type='volume' was not translated the
following error would occur:

 $ virsh start VM
 error: Failed to start domain VM
 error: An error occurred, but the cause is unknown

After this patch:

 $ virsh start VM
 error: Failed to start domain VM
 error: internal error: storage source pool 'tmp' volume 'pull3.qcow2' is not translated

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

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