bhyve: error out if fwcfg user file isn't read completely

At the moment, fwcfg reads the file once at startup and passes these
data to the guest. Therefore, we should always read the whole file.
Otherwise we should error out.

Additionally, GCC12 complains that the comparison whether
fwcfg_file->size is lower than 0 is always false due to the limited
range of data type.

Reviewed by: markj
Fixes: ca14781c8170f3517ae79e198c0c880dbc3142dd ("bhyve: add cmdline option for user defined fw_cfg items")
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40076