fix parse_*() do not take argument

Bug recently introduced with commit bbbdd6b592.
add libvirt-python binding in install_requires

The binding is now available in PyPI so the dependency can now be
declared safely.
check disks to overwrite in boot_vm()

All disks in the list given in parameter of Cloubed.boot_vm() are
checked. If the domain does not have such disk, remove this disk and log
warning message.
check networks to recreate in boot_vm()

All networks in the list given in parameter of Cloubed.boot_vm() are
checked. If the domain is not connected to such network, discard this
network and log warning message.
move API functions from cloubed.py to __init__.py

External scripts should replace:
from cloubed import cloubed

With:
import cloubed

Examples have been updated to reflect this change.

Fixes #18.
launch event manager thread only if needed

The event manager thread is only needed when Cloubed.wait_event() is
called. The thread is therefore now launched in this method.