bhyve: increase SNAPSHOT_BUFFER_SIZE

After commit fb51ddb20d57, suspending a vm fails with the error:

vm_snapshot_buf: buffer too small

Sponsored by: vStack
Reviewed by: markj, rew
Fixes: fb51ddb20d57 ("bhyve: increase fbuf display resolution limit")
Differential Revision: https://reviews.freebsd.org/D43218