tests: allow setting extra capabilities on top of versioned caps

tests: allow setting extra capabilities on top of versioned caps

In the QEMU tests a test can either use an explicitly listed set of
capabilities, or can request those matching a particular QEMU
version. Sometimes it is desirable to be able to list extra caps
on top of those implied by a particular version.

This is useful, for example, when QEMU won't report certain features
unless it was run on particular hardware or kernels, and those were
not used when a caps snapshot was imported to the libvirt source tree.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>