bhyve: Remove vmctx member from struct vm_snapshot_meta.

This is a userland-only pointer that isn't relevant to the kernel and
doesn't belong in the ioctl structure shared between userland and the
kernel. For the kernel, the old structure for the ioctl is still
supported under COMPAT_FREEBSD13.

This changes vm_snapshot_req() in libvmmapi to accept an explicit
vmctx argument.

It also changes vm_snapshot_guest2host_addr to take an explicit vmctx
argument. As part of this change, move the declaration for this
function and its wrapper macro from vmm_snapshot.h to snapshot.h as it
is a userland-only API.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D38125