python: set default value to optional arguments

python: set default value to optional arguments

When prefixing with string (optional) or optional in the description
of arguments to libvirt C APIs, in python, these arguments will be
set as optional arugments, for example:

 * virDomainSaveFlags:
 * @domain: a domain object
 * @to: path for the output file
 * @dxml: (optional) XML config for adjusting guest xml used on restore
 * @flags: bitwise-OR of virDomainSaveRestoreFlags

 the corresponding python APIs is
 restoreFlags(self, frm, dxml=None, flags=0)

The following python APIs are changed to:
 blockCommit(self, disk, base, top, bandwidth=0, flags=0)
 blockPull(self, disk, bandwidth=0, flags=0)
 blockRebase(self, disk, base, bandwidth=0, flags=0)
 migrate(self, dconn, flags=0, dname=None, uri=None, bandwidth=0)
 migrate2(self, dconn, dxml=None, flags=0, dname=None, uri=None, bandwidth=0)
 migrateToURI(self, duri, flags=0, dname=None, bandwidth=0)
 migrateToURI2(self, dconnuri=None, miguri=None, dxml=None, flags=0, \
               dname=None, bandwidth=0)
 saveFlags(self, to, dxml=None, flags=0)
 migrate(self, domain, flags=0, dname=None, uri=None, bandwidth=0)
 migrate2(self, domain, dxml=None, flags=0, dname=None, uri=None, bandwidth=0)
 restoreFlags(self, frm, dxml=None, flags=0)