Original passport is really not necessary for
processing your work visa..work visa is a single
entry visa which an employer can obtain it on your
behalf which used to be valid for 60 days.

After u arrive in qatar the process of your id start
with medical finger prints and immigration etc..

For all these process company will take u along with
your original passport.

after you finish the medical and finger prints they
will post your Residence permit in ur passport.
ID is just a card which you need keep with you
anywhere you go outside. If police ask you you have
to show the id.

After ur residence permit is ready you cant leave
this country without an exit permit from your
sponsor so even if you have passport with you it is
not really useful. If you think your passport is not safe in ur home or u afraid of loosing it then its
better your employer have it in their safety lockers.