qemuMigrationSrcNBDStorageCopy: Don't pass migrate_flags

qemuMigrationSrcNBDStorageCopy: Don't pass migrate_flags

'migrate_flags' can be updated in the only caller and since
qemuMigrationSrcNBDStorageCopy already takes @flags which contains
VIR_MIGRATE_NON_SHARED_INC (used to set
 QEMU_MONITOR_MIGRATE_NON_SHARED_INC) we can completely remove the
parameter.

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