bhyveload: make error printing consistent

Previously we used a mix of perror(3) + exit(3) and err(3); standardize
on the latter instead. This does remove one free() in an error path,
because we're decidedly leaking a lot more than just the loader name
there (loader handle, vcpu, vmctx...) anyways.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43331