qemuDomainAttachDeviceMknodHelper: Remove symlink before creating it

qemuDomainAttachDeviceMknodHelper: Remove symlink before creating it

https://bugzilla.redhat.com/show_bug.cgi?id=1528502

So imagine you have /dev/blah symlink which points to /dev/sda.
You attach /dev/blah as disk to your domain. Libvirt correctly
creates the /dev/blah -> /dev/sda symlink in the qemu namespace.
However, then you detach the disk, change the symlink so that it
points to /dev/sdb and tries to attach the disk again. This time,
however, the attach fails (well, qemu attaches wrong disk)
because the code assumes that symlinks don't change. Well they
do.

This is inspired by test fix written by Eduardo Habkost.

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