qemu: Support setting the 'removable' flag for USB disks

qemu: Support setting the 'removable' flag for USB disks

Add an attribute named 'removable' to the 'target' element of disks,
which controls the removable flag. For instance, on a Linux guest it
controls the value of /sys/block/$dev/removable. This option is only
valid for USB disks (i.e. bus='usb'), and its default value is 'off',
which is the same behaviour as before.

To achieve this, 'removable=on' (or 'off') is appended to the '-device
usb-storage' parameter sent to qemu when adding a USB disk via
'-disk'. A capability flag QEMU_CAPS_USB_STORAGE_REMOVABLE was added
to keep track if this option is supported by the qemu version used.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=922495
Signed-off-by: Peter Krempa <pkrempa@redhat.com>