Description:
Previously qt was an optional dependency of virtualbox, but now, starting with 5.1.2-1 it is required and wants to install a lots of packages on a headless system.

Additional info:
- in 5.0.* qt4 was an optional dependency
- in 5.1.0-1 it was replaced by qt5-x11extras [0]
- starting with 5.1.2-1 qt55-base added, and both qt5-base and qt5-x11extras are depends instead of optdepends
- upgraded to 5.1.2-3 using --assume-installed=qt5-base,qt5-x11extras and the headless functionality is working perfectly fine

Steps to reproduce:
sudo pacman -S virtualbox
resolving dependencies...
:: There are 4 providers available for libgl:
:: Repository extra
1) mesa-libgl 2) nvidia-304xx-libgl 3) nvidia-340xx-libgl 4) nvidia-libgl

Enter a number (default=1):
looking for conflicting packages...
warning: dependency cycle detected:
warning: harfbuzz will be installed before its freetype2 dependency

Packages (39) damageproto-1.2.1-3 elfutils-0.166-1 fontconfig-2.12.1-3
freetype2-2.6.5-2 graphite-1:1.3.8-1 harfbuzz-1.3.0-1
icu-57.1-1 inputproto-2.3.2-1 libdrm-2.4.70-1 libevdev-1.5.3-1
libgudev-230-1 libinput-1.4.1-1 libjpeg-turbo-1.5.0-1
libomxil-bellagio-0.9.3-1 libpciaccess-0.13.4-1
libproxy-0.4.12-2 libtxc_dxtn-1.0.1-6 libwacom-0.22-1
libxdamage-1.1.4-2 libxi-1.7.6-1 libxkbcommon-0.6.1-1
libxkbcommon-x11-0.6.1-1 libxshmfence-1.2-1 libxxf86vm-1.1.4-1
llvm-libs-3.8.1-1 mesa-12.0.1-7 mesa-libgl-12.0.1-7
mtdev-1.1.5-1 qt5-base-5.7.0-2 qtchooser-53-2 tslib-1.1-1
wayland-1.11.0-1 xcb-util-0.4.0-1 xcb-util-image-0.4.0-1
xcb-util-keysyms-0.4.0-1 xcb-util-wm-0.4.1-1
xf86vidmodeproto-2.3.1-3 xkeyboard-config-2.18-1
virtualbox-5.1.4-1

Total Download Size: 63.31 MiB
Total Installed Size: 286.46 MiB
Net Upgrade Size: 193.37 MiB

https://git.archlinux.org/svntogit/community.git/diff/trunk/PKGBUILD?h=packages/virtualbox&id=d68583789ab95db0ded09beaf135456940570fbe