qemu: migration: Use correct flag when preparing block dirty bitmaps

qemu: migration: Use correct flag when preparing block dirty bitmaps

qemuMigrationSrcRunPrepareBlockDirtyBitmaps receives the flags parameter
from qemuMigrationSrcRun, where flags are based on the main API enum
values. Similar to commit f58349c9c6, use the main API enum instead of
internal driver enum when checking flags in
qemuMigrationSrcRunPrepareBlockDirtyBitmaps.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>