bhyve: generate NVRAM bhyve arguments

bhyve: generate NVRAM bhyve arguments

Currently, bhyve bootrom specification looks like this:

 bhyve ... -l bootrom,/path/to/firmware.fd

In addition to that, it supports specifying the VARS files using:

 -l bootrom,/path/to/firmware.fd,/path/to/my_domain_VARS.fd

Update virBhyveProcessBuildBhyveCmd() to include the VARS file if NVRAM
is specified in the domain XML.

Additionally, support copying this file from the specified template. To
do that, introduce the bhyveProcessPrepareHost() and related helpers.
They are currently not doing anything but NVRAM preparations, but should
be useful for other host-side related tasks in the future.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>