bhyve: Fix handling of -r

Just make "restore_file" a global variable so that it can be set by the
MD option handler.

Reviewed by: corvink
Reported by: bdrewery
Fixes: 981f9f7495bb ("bhyve: Push option parsing down into bhyverun_machdep.c")
Differential Revision: https://reviews.freebsd.org/D44974