vfio: selftests: Allow passing multiple BDFs on the command line

vfio: selftests: Allow passing multiple BDFs on the command line

Add support for passing multiple device BDFs to a test via the command
line. This is a prerequisite for multi-device tests.

Single-device tests can continue using vfio_selftests_get_bdf(), which
will continue to return argv[argc - 1] (if it is a BDF string), or the
environment variable $VFIO_SELFTESTS_BDF otherwise.

For multi-device tests, a new helper called vfio_selftests_get_bdfs() is
introduced which will return an array of all BDFs found at the end of
argv[], as well as the number of BDFs found (passed back to the caller
via argument). The array of BDFs returned does not need to be freed by
the caller.

The environment variable VFIO_SELFTESTS_BDF continues to support only a
single BDF for the time being.

Reviewed-by: Alex Mastro <amastro@fb.com>
Tested-by: Alex Mastro <amastro@fb.com>
Reviewed-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20251126231733.3302983-4-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>