Provides a qemu-system-x86 virtual package

Provides a qemu-system-x86 virtual package

This allows adding additional debian packages on pve hosts
which depends on a qemu binary being installed.

The following install paths were tested:

 * install of proxmox-ve on debian systems with standard package
 set:  works as expected

 * install of proxmox-ve on debian having the debian qemu package
 installed: refused by the package manager as expected, until
 qemu, qemu-system-x86, libspice-server1 are deinstalled

 * manual install of qemu-system-x86 after proxmox-ve
 is installed: refused by the package manager as expected, until
 libspice-server1 is installed. If libspice-server1 is installed
 then proxmox-ve is deinstalled.

 * install of packages depending on qemu-system-x86
 having proxmox-ve installed: thoses packages are installed as
 expected, no conflict

Acked-by: Wolfgang Bumiller <w.bumiller@proxmox.com>