openvz: fixed two memory leaks on migration code

openvz: fixed two memory leaks on migration code

The first one occurs in openvzDomainMigratePrepare3Params() where in
case no remote uri is given, the distant hostname is used. The name is
obtained via virGetHostname() which require callers to free the
returned value.
The second leak lies in openvzDomainMigratePerform3Params(). There's a
virCommand used later. However, at the beginning of the function
virCheckFlags() is called which returns. So the command created was
leaked.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>