meson: refine check for whether to look for virglrenderer

meson: refine check for whether to look for virglrenderer

The check should be performed even if !have_system, as long as there is some hope that
vhost-user-gpu will be built.  Store into have_vhost_user_gpu whether vhost-user-gpu
will be built; we will also use the variable to decide whether to look for libepoxy.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>