block.c: adding bdrv_co_delete_file

Using the new 'bdrv_co_delete_file' interface, a pure co_routine function
'bdrv_co_delete_file' inside block.c can can be used in a way similar of
the existing bdrv_create_file to to clean up a created file.

We're creating a pure co_routine because the only caller of
'bdrv_co_delete_file' will be already in co_routine context, thus there
is no need to add all the machinery to check for qemu_in_coroutine() and
create a separated co_routine to do the job.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20200130213907.2830642-3-danielhb413@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>