vbox: save the guest FPU state before 'longjmp()'

'longjmp()' restores the (partial) FPU state saved by 'setjmp()', so it's
necessary to save the guest FPU state before calling 'longjmp()'.

Fixes #1282
vbox: always transfer FPU ownership back to the VM

When the 'Mtd::FPU' flag is set during the registration of a
virtualization event handler, it must also be set whenever the event
handler returns.

Fixes #1283
vbox: set the 'CPUM_CHANGED_GLOBAL_TLB_FLUSH' flag

Fixes #1281