m4: bhyve: Fix check for the required bhyve programs
    
    bhyveload and bhyvectl wouldn't be checked otherwise as the configure
    script wouldn't execute one of the tests:
    
    checking for bhyve... /usr/local/sbin/bhyve
    checking for bhyvectl... /usr/local/sbin/bhyvectl
    checking for bhyveload... /usr/local/sbin/bhyveload
    ./configure: line 62602: test: too many arguments
    
    Fix the shell statement testing the 3 binaries.