qemu: domain: Initialize FD passthrough for a virStorageSource before using it

qemu: domain: Initialize FD passthrough for a virStorageSource before using it

The call to 'qemuBlockStorageSourceNeedsFormatLayer()' bases the
decision also on the state of the passed FD, so we must initialize the
passthrough data via 'qemuDomainPrepareStorageSourceFDs()' before the
aforementioned call.

In the test change it's visible that we didn't add the necessary 'raw'
driver which allows the 'protocol' blockdev to be opened in 'rw' mode so
that qemu picks the proper file descriptior while keeping the device
read-only.

Resolves: https://issues.redhat.com/browse/RHEL-37519
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>