qemu: capabilities: Disable detection of QEMU_CAPS_BLOCK_EXPORT_ADD

qemu: capabilities: Disable detection of QEMU_CAPS_BLOCK_EXPORT_ADD

We use the capability to switch to using 'block-export-add' in the
upcoming qemu release instead of the at the same time deprecated
'nbd-server-add'.

Unfortunately qemu wants to change the interface of 'block-export-add'
before the release. Since we've tried to stay up to date and added the
code before it was written in stone, we need to disable the use of the
new interface for the upcoming libvirt release so that we don't have a
version of libvirt which would not work with the upcoming qemu version.

Remove the detection of 'block-export-add' until we are more sure how
the qemu interface will look.

This patch partially reverts commit adb9f7123adb94645

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>