Shrink some of the longer lines in xapi_vm_migrate.ml

Signed-off-by: John Else <john.else@citrix.com>
Move a comment to where it makes sense

Signed-off-by: John Else <john.else@citrix.com>
Tidy module imports/aliases

Signed-off-by: John Else <john.else@citrix.com>
Refactor out common wait-for-fist-point logic

Signed-off-by: John Else <john.else@citrix.com>
Move some SXM-specific code into Xapi_sxm

Signed-off-by: John Else <john.else@citrix.com>
Variable rename for clarity

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

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