qemu: fs: do not try to fill binary path if we have a socket

qemu: fs: do not try to fill binary path if we have a socket

We do not need to look for a suitable binary in the vhost-user
description files, if we aren't the ones starting it.
Otherwise startup will fail with:

error: Failed to start domain 'vm1'
error: operation failed: Unable to find a satisfying virtiofsd

https://bugzilla.redhat.com/show_bug.cgi?id=1855789

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>