bhyvectl.8: Polish

+ Add how to find open VMs in the example, linking apropos Pa=/dev/vmm
+ Move note about additional bhyve options after options for flow
+ Reclaim some space by shortening <filename> to <file>
+ Align options list width, tested at MANWIDTH 59 and 80
+ Remove unnecessary roff quoting and tag SPDX

MFC after: 3 days
Reviewed by: carlavilla, markj, mhorne
Approved by: carlavilla, mhorne (mentors)
Differential Revision: https://reviews.freebsd.org/D48401