qcow2: Let qcow2_create() handle protocol layer

qcow2: Let qcow2_create() handle protocol layer

Currently, qcow2_create() only parses the QemuOpts and then calls
qcow2_co_create() for the actual image creation, which includes both the
creation of the actual file on the file system and writing a valid empty
qcow2 image into that file.

The plan is that qcow2_co_create() becomes the function that implements
the functionality for a future 'blockdev-create' QMP command, which only
creates the qcow2 layer on an already opened file node.

This is a first step towards that goal: Let's move out anything that
deals with the protocol layer from qcow2_co_create() into
qcow2_create().  This means that qcow2_co_create() doesn't need a file
name any more.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>