Add virFDStreamOpenPTY() function which is a wrapper around
virFDStreamOpenFileInternal() with putting the device it opens into a
raw mode.

Make virChrdevOpen() use virFDStreamOpenPTY() for
VIR_DOMAIN_CHR_TYPE_PTY devices.

This fixes mangled console output when libvirt runs on FreeBSD as it
requires device it opens to be placed into a raw mode explicitly.