qemu: Introduce qemuMigrationParty enum

qemu: Introduce qemuMigrationParty enum

Some migration parameters and capabilities are supposed to be set on
both sides of migration while others should only be set on one side. For
example, CPU throttling parameters make no sense on the destination and
they can be used even if the destination is too old to support them.

To make qemuMigrationParamsFromFlags more general and usable on both
sides of migration, we need to tell it what side it's been called on.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>