vmd(8): zero consdev in bootargs to fix booting ramdisks

vmd(8): zero consdev in bootargs to fix booting ramdisks

Mischa Peters reported that booting a bsd.rd from 7.2 or newer
stopped working with vmd(8) in 7.2.

Direct booting kernels requires vmd to build boot args in guest
memory. Recently, the bios_consdev_t struct changed in amd64
machdep.c, adding additional struct members. vmd wasn't zeroing out
the struct, causing the booted kernel to read garbage.

While here, cleanup some of push_bootargs to use descriptive names
for boot args and standardize on explicit usage of uint32_t.

ok claudio, mlarkin