Fix bugs in virDomainMigrate v2 code.

Fix bugs in virDomainMigrate v2 code.

Paolo Bonzini points out that in my refactoring of the code for
virDomainMigrate(), I added a check for the return value from
virDomainMigratePerform().  The problem is that we don't want to
exit if we fail, we actually want to go on and do
virDomainMigrateFinish2() with a non-0 return code to clean things
up.  Remove the check.

While reproducing this issue, I also noticed that we wouldn't
always properly propagate an error message.  In particular, I
found that if you blocked off the migration ports (with iptables)
and then tried the migration, it would actually fail but we would
get no failure output from Qemu.  Therefore, we would think we
succeeded, and leave a huge mess behind us.  Execute the monitor
command "info migrate", and look for a failure string in there
as well.

Signed-off-by: Chris Lalancette <clalance@redhat.com>