Add remote SR/VDI to mirror_record type

Signed-off-by: John Else <john.else@citrix.com>
CA-78901: Fix snapshot info for SXM'ed snapshot disks

Once we have copied all snapshot disks to the remote SR and mirrors are
running for all live VDIs, rebuild the snapshot tree on the remote SR
for each live VDI.

Signed-off-by: John Else <john.else@citrix.com>