VirtualBox 5.1.0 RC1

# The following major new features were added:
* VMM: new APIC and I/O APIC implementations that result in significantly improved per-formance in certain situations (for example with networking, bug #15295)
* VMM: added support for Hyper-V paravirtualized debugging of Windows guests
* VMM: emulate even more MMIO and shadow pagetable exits without going back to user mode
* GUI: overall migration to Qt5
* GUI: passive API event listener improving the VM GUI performance and response time
* Audio: added HDA (High Definition Audio) support for newer Linux guests
* Audio: added on-demand timers which should improve the overall performance and reduce the CPU consumption
* better support for Python 3
# In addition, the following items were fixed and/or added:
* VMM: many fixes
* GUI: the Detach UI action is now a part of the VM Machine menu
* GUI: reworked session information window
* GUI: the new VM wizard now allows to choose the VM location for the VM which is being created
* GUI: fixed location of touch events in unscaled HiDPI mode (bugs #105381, #14366)
* GUI: various bugfixes and internal cleanup
* Storage: use fallocate or similar if present to speed up creating fixed-sized disk images
* Storage: implemented NVMHCI-compatible storage controller
* Audio: added support for master volume controls in the AC’97 emulation
* Audio: various bugfixes and infrastructure improvements
* EFI: many improvements, for example allow to boot from USB
* OVF: improved handling of signed manifests
* Windows hosts: rework COM proxy (e.g. bug #8298)
* Linux installers: no longer rely on DKMS for module rebuilding