https://bugzilla.redhat.com/show_bug.cgi?id=1632833

When doing a SCSI passthrough we don't put format= onto the
command line. This causes qemu to probe the format automatically
which ends up in a warning in the domain log and possible qemu
disabling writes to the first block (according to the warning
message).

Based-on-work-of: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>