Add qemuVhostUserFetchConfigs() to discover vhost-user helpers.

qemuVhostUserFillDomainGPU() will find the first matching GPU helper
with the required capabilities and set the associated
vhost_user_binary.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>