Use '-e' to check if the virtio backing file has already been created.

The '-f' check works fine on a regular file but not if the backing file is
a device (e.g., /dev/md0). In this case it would print a misleading but
otherwise benign message about the backing file not being present.

Submitted by:	Marcus Reid (marcus@blazingdot.com)
Discussed with:	grehan
Make automount(8)/automountd(8) treat percent sign as a valid part
of path.  It's useful for spaces encoded as %20 as msdosfs labels.

Submitted by:	glebius@
MFC after:	1 month
Emulate "POP r/m".

This is needed to boot OpenBSD/i386 MP kernel in bhyve.

Reported by:	grehan
MFC after:	1 week