vmm: Destroy character devices synchronously.

This fixes a userland race where bhyveload or bhyve can fail to reuse
a VM name after bhyvectl --destroy has returned.

Reported by: Michael Dexter
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D35186

(cherry picked from commit 7a0c23da4eaa63f00e53aa18f3ab1f2bb32f593a)