hw/s390x/ipl: Provide more memory to the s390-ccw.img firmware

hw/s390x/ipl: Provide more memory to the s390-ccw.img firmware

We are going to link the SLOF libc into the s390-ccw.img, and this
libc needs more memory for providing space for malloc() and friends.
Thus bump the memory size that we reserve for the bios to 3 MiB
instead of only 2 MiB. While we're at it, add a proper check that
there is really enough memory assigned to the machine before blindly
using it.

Message-ID: <20240621082422.136217-3-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>