block/raw-posix: Strip protocol prefix on creation

The hdev_create() implementation in block/raw-posix.c is used by the
"host_device", "host_cdrom" and "host_floppy" protocol block drivers
together. Thus, any of the associated prefixes may occur and exactly one
should should be stripped, if it does (thus,
"host_device:host_cdrom:/dev/cdrom" is not shortened to "/dev/cdrom").

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>