sun4uv: fix serial initialization regression

Since commit b6607a1a204d, serial_hds_isa_init() was introduced to
factor out serial_isa_init() loops. However, sun4uv shouldn't start from
0 when there is a mm serial on 0 already. Add a "from" argument to
serial_hds_isa_init().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-5-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
