fix(toolbox): fix a ramfs detection bug in xen-create

Before the fix, for ramfs with the Ubuntu format, the ramfs file could
be found but was stored in the wrong variable and so was considered as
not found.
Now it is stored in the right variable.
fix(toolbox): modify the size computation of VM root partition

Before, the root partition size was computed from the total size of
the host root partition.
Now it is computed from the used size of the host root partition.
feat(toolbox): add an exclusion mechanism for xen-create

Add a way to pass as last argument to xen-create, the directories to
exclude of the rsync synchronization.