qemu i386 used to support more than 4GB of RAM through PAE, but it has
been disabled for an unknown reason. Reenable it.

Note that simply running qemu x86_64 and emulating a 32-bit CPU is not
a solution to this problem as it is about 15% slower (it needs to
emulate 64 bit registers even if half of them are not used). On the
other hand, I haven't seen any measurable impact by switching
target_phys_bits to 64.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>