bhyve: fix arguments to ioctl(VMIO_SIOCSIFFLAGS)

ioctl(2)'s with integer argument shall pass command argument by value,
not by pointer. The ioctl(2) manual page is not very clear about that.
See sys/kern/sys_generic.c:sys_ioctl() near IOC_VOID.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D42366
Fixes: fd8b9c73a5a63a7aa438a73951d7a535b4f25d9a