qemu: don't try to query QEMU about migration blockers during offline migration

qemu: don't try to query QEMU about migration blockers during offline migration

The new code that queries QEMU about migration blockers was put at the
top of qemuMigrationSrcIsAllowed(), but that function can also be
called in the case of offline migration (ie when the domain is
inactive / QEMU isn't running). This check should have been put inside
the "if (!(flags & VIR_MIGRATE_OFFLINE))" conditional, so let's move
it there.

Fixes: 156e99f686690855be4e45d9b8b3194191a8bc31
Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>