MFC 325727:
bhyve: avoid applying capsicum capabilities to file that was not opened

When using -l option targeting file that can't be opened (ie. nmdm module
is not loaded and /dev/nmdm* is specified) bhyve tries to apply capsicum
capabilities to a file that was not opened.

Enclose that code in an if statement and only run it on correctly opened
descriptor also providing meaningful message in case of an error.