vbox_snapshot_conf: fix wrong use of 'xmlSaveFormatFileEnc'

vbox_snapshot_conf: fix wrong use of 'xmlSaveFormatFileEnc'

The function 'xmlSaveFormatFileEnc' has a last option to set
if you want to format dumped xml with whitespaces or not.

Older libxml2, the one used in RHEL6, take this option as it is
but newer libxml2 check this option if it's true or not. This
small difference somehow makes things messy on RHEL6 and generated
xml had extra new line and extra whitespaces.

We should pass 1 instead if -1 because the -1 confuses the libxml2.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>