bsdinstall: Filter out devices that cannot be opened

Devices that cannot be opened are most likely the install media and
should not be listed as destinations.

Reviewed by: allanjude
Approved by: allanjude
Differential Revision: https://reviews.freebsd.org/D34879
Sponsored by: Rubicon Communications, LLC ("Netgate")