bhyve: fix regression in legacy virtio-9p config parsing

Commit 621b5090487de9fed1b503769702a9a2a27cc7bb introduced a regression
in legacy virtio-9p config parsing by not initializing *sharename to
NULL. As a result, "sharename != NULL" check in the first iteration fails
and bhyve exits with "virtio-9p: more than one share name given".

Fix by adding NULL back.

Approved by: grehan

(cherry picked from commit f2ecc0d1b7d569b4b2c930c7450390c04778ee8a)