hotplug: Update block-tap

Implement a sharing check like the regular block script.

Checking tapback inside block-tap is too late since it needs to be
running to transition the backend to InitWait before block-tap is run.

tap-ctl check will be removed when the requirement for the blktap kernel
driver is removed.  Remove it now as it is of limited use.

find_device() needs to be non-fatal allow a sharing check.

Only write physical-device-path because that is all that tapback needs.
Also write_dev doesn't handled files and would incorrectly store
physical-device as 0:0 which would confuse the minor inside tapback

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>