virsh: improve doMigrate function docs

virsh: improve doMigrate function docs

When running virsh migrate with --xml option and actual xml file doesn't
exist, virsh hasn't output any error information, although return value
is 1.

* tools/virsh.c: Raising a appropriate error information when operation fails.

* How to reproduce?
% virsh migrate <domain> --live qemu+ssh://<target host>/system --xml non-existent.xml
% echo $?

* Fixed result:
error: file 'non-existent.xml' doesn't exist

Signed-off-by: Alex Jia <ajia@redhat.com>