bhyve: Refactor vmexit_suspend() a bit

Move some of its logic into fbsdrun_deletecpu(). This makes it easier
to split vmexit handlers into a separate file, which in turn makes
landing arm64 support easier. Also increase the scope of the mutex and
use it to synchronize updates to the vcpu mask. No functional change
intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40573