ccid-card-passthru, dev-smartcard-reader: add debug environment variables

Introduces a new utility function: parse_debug_env to avoid code
duplication.

This overrides whatever debug value is set on the corresponding devices
from the command line, and is meant to ease the usage with any
management stack. For libvirt you can set environment variables by
extending the dom namespace, i.e:

<domain type='kvm' id='3' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <qemu:commandline>
    <qemu:env name='QEMU_CCID_PASSTHRU_DEBUG' value='4'/>
    <qemu:env name='QEMU_CCID_DEBUG' value='4'/>
  </qemu:commandline>
</domain>

Signed-off-by: Alon Levy <alevy@redhat.com>
Reviewed-by: Marc-André Lureau <mlureau@redhat.com>
