qemuMigrationSrcRun: Sanitize setting of cookieFlags and migrate_flags on storage...

qemuMigrationSrcRun: Sanitize setting of cookieFlags and migrate_flags on storage migration

Base the decision on the main API flags (VIR_MIGRATE_NON_SHARED_DISK,
QEMU_MONITOR_MIGRATE_NON_SHARED_INC) via a boolean 'storageMigration'
rather than juggling everything trhough 'migration_flags'.

After this patch 'migration_flags' is updated to contain the legacy
storage migration flags only when we'll be about to use it rather than
setting it and then resetting it.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Tested-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>