bhyve: Fix synopsis in the usage message

In particular:

• Sort short options to align with style(9)
• Add two missing flags: -G and -r
• Drop unnecessary angle brackets for consistency
• Rename the "vm" argument to vmname for consistency with the manual page

MFC after: 2 weeks

(cherry picked from commit 03c3e5e40d6497afa33df1d0b43857157c086729)

Note: the -r flag is not present on branch stable/12.