Add a migration type global hypervisor parameter

Since migration live/non-live is more stable (e.g.) for Xen-PVM versus
Xen-HVM, we introduce a new parameter for what mode we should use by
default (if not overridden by the user, in the opcode).

The meaning of the opcode 'live' field changes from boolean to either
None (use the hypervisor default), or one of the allowed migration
string constants. The live parameter of the TLMigrateInstance is still a
boolean, computed from the opcode field (which is no longer passed to
the TL).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>