target-i386: set target_phys_bits to 64

target-i386: set target_phys_bits to 64

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>