hw/mips: Add API to convert KVM guest KSEG0 <-> GPA

hw/mips: Add API to convert KVM guest KSEG0 <-> GPA

Add API for converting physical addresses to KVM guest KSEG0 addresses,
and fix the existing API for converting KSEG0 addresses to physical
addresses to work in the KVM case. Both have the same sized KSEG0, so
it's just a case of fixing the mask.

In KVM trap and emulate mode both the guest kernel and guest userspace
execute in useg:
    Guest User address space:   0x00000000..0x3fffffff
    Guest Kernel Unmapped:      0x40000000..0x5fffffff
    Guest Kernel Mapped:        0x60000000..0x7fffffff

Signed-off-by: Sanjay Lal <sanjayl@kymasys.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>