doc: qemu: fix and update documentation

doc: qemu: fix and update documentation

The current URL for QEMU_EFI.fd is not found. Update the link to
point to the new one.

If you run the shell command as instructed, you will see this error:
  qemu-system-aarch64: keep_bootcon: Could not open 'keep_bootcon': No such file or directory

The part "console=ttyAMA0,38400 keep_bootcon root=/dev/vda2" is the
kernel parameter, so it must be quoted.

As of writing, QEMU v4.2.0 is the latest, but it does not work for
TF-A (It has been fixed in the mainline.) QEMU v4.1.0 works fine.

With those issues addressed, I succeeded in booting the latest kernel.

Tested with QEMU v4.1.0 and Linux 5.5 (defconfig with no modification).
Update the tested versions.

Change-Id: Ic85db0e688d67b1803ff890047d37de3f3db2daa
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>