bhyve/snapshot: rename and bump size of MAX_SNAPSHOT_VMNAME

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 - this file is used
by the save/restore feature.

Since the file doesn't have anything to do with a vm name, rename
MAX_SNAPSHOT_VMNAME to MAX_SNAPSHOT_FILENAME. Bump the size to PATH_MAX
while here.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D28879