slirp: Reorder initialization

slirp: Reorder initialization

This patch reorders the initialization of slirp itself as well as its
associated features smb and redirection. So far the first reference to
slirp triggered the initialization, independent of the actual -net user
option which may carry additional parameters. Now we save any request to
add a smb export or some redirections until the actual initialization of
the stack. This also allows to move a few parameters that were passed
via global variable into the argument list of net_slirp_init.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>