Fix tools/boot/ci-qemu-test.sh and make some improvements

Update ci-qemu-test.sh

o Update the path to the OVMF file, which is now in /usr/local/share/uefi-edk2-qemu.
o Use the more modern q35, pc-q35-3.0 (Q35 + ICH9, 2009) QEMU machine

instead of the default, obsolete pc, pc-i440fx-3.0 (i440FX + PIIX, 1996).
For example this adds ACPI support.

o Specify the system firmware using the newer pflash drive syntax instead

of bios.

o Remove extra, unneeded devices by passing -nodefaults.
o Change text to talk about 'firmware' instead of 'bios', since UEFI

isn't a BIOS.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D20074