bhyve/snapshot: rename and bump size of MAX_SNAPSHOT_VMNAME

[rew]

Summary

MAX_SNAPSHOT_VMNAME is a macro used to set the size of a character
buffer that stores a filename or the path to a file. I suggest
renaming this macro to MAX_SNAPSHOT_FILENAME instead.

Since MAX_SNAPSHOT_FILENAME is used to store a filename, NAME_MAX is
probably a better value. I thought about PATH_MAX, as it may be more
appropriate, but I chose the more conservative value of the two, so
NAME_MAX.

[jhb]

I would probably use PATH_MAX. Another option is that once this socket is a datagram socket, this could become a variable length packet with a VLA for the filename instead of a fixed size buffer.