bhyve: Register hlt and pause vmexit handlers unconditionally

These exit handlers might not be used if the corresponding VM
capabilities are not set, but there is no harm in putting them into the
handler table regardless. Doing so simplifies initialization code,
makes it easier to split vmexit handlers into a separate file, and lets
us declare the handler table as const.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40572