Add possibility to read password from stdin

[megabert]

I'd like to have the possibility to have the password read from stdin to reduce the danger for the password to be easily captured when having root access to a XenServer.

At the moment the password is visible in the process list when directly supplied at the command line or nearly cleartext in the specified file base64-encoded.

That added change adds a third possibility: Specify "-" as the password in which case the password is read from stdin like this:

VmBackup.py - ".*" <<<'secret_password'

or

echo "$password" | ssh xenserver VmBackup.py - "vm-name-to-backup"

In this way the password is not shown in the process list and it does not have to be present on all XenServers. It can be kept on a secure remote machine, which initiates the start of the script.

NOTE:
I'm not really familiar with python very well, so look at my code with care.