qemu: block: Extract formatting of 'driver' attribute from child formatters
qemu: block: Extract formatting of 'driver' attribute from child formatters

To allow reusing the formatters in the code for creating JSON properties
for 'blockdev-create' we need to create everything except the 'driver'
attribute.

Use the new helper virJSONValueObjectPrependString to put the driver at
the same place so that we don't change any output.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

olafhering/libvirt,jfehlig/libvirt,eskultety/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jardasgit/libvirt,eskultety/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,crobinso/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,olafhering/libvirt,nertpinx/libvirt,eskultety/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,nertpinx/libvirt,nertpinx/libvirt,eskultety/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,eskultety/libvirt,fabianfreyer/libvirt,crobinso/libvirt,libvirt/libvirt,fabianfreyer/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jardasgit/libvirt