Allow xe vm-snapshot be run with multiple vms.

xe help vm-snapshot documents that this is how the command should
behave. Looking back through the `blame' of this line doesn't seem to
indicate any reason to disallow this.
Replace hard-coded mail-alarm sender with a user-setable value.

Without this patch, email from the pool are sent from
noreply@${FSQN}. Some organisations setup their email so as to only
allow emails from  whitelist of addresses, so are unable to get mail
alerts.

This patch lets the administrator run:
xe pool-param-set other-config:mail-sender=${SENDER}
to choose an whitelisted address.
Add missing () to call to getfqdn in mail-alarm
Merge pull request #1456 from oc243/mail-alert-change-sender

Replace hard-coded mail-alarm sender with a user-setable value.
Merge pull request #1455 from oc243/master

Allow xe vm-snapshot be run with multiple vms.