https://bugzilla.redhat.com/show_bug.cgi?id=1191355

When we attempt to migrate a vm with a migrateuri that has no scheme:

 # virsh migrate test4 --live qemu+ssh://lhuang/system --migrateuri 127.0.0.1

target libvirtd will crash because uri->scheme is NULL in
qemuMigrationPrepareDirect on this line:

     if (STRNEQ(uri->scheme, "tcp") &&

Add a value check before this line. Also fix a bug like this in
doNativeMigrate, that could only happen when destination libvirtd
returned an incorrect URI.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>